ArticlePDF Available

A Theoretical Multiscale Approach to Study the Initial Steps Involved in the Chemical Reactivity of Soot Precursors

Authors:

Abstract

In the present study, bond formation reactions between soot precursors and their role in the soot inception process are investigated. The soot precursors were generated in macroscopic detailed gas-phase kinetic calculations and according to certain criteria introduced in simulation boxes to model bond formation between soot precursor molecules with reactive force field molecular dynamics modeling. The impacts of temperature, fuel mixture, and equivalence ratio have been investigated on the rate and structure of the newly formed molecules. The resulting structures compare well to previously reported experimental results. Furthermore, the bond formation rate between PAHs is found to be linearly correlated with the temperature at which the PAH precursors are generated, while fuel and equivalence ratio do not have a direct impact on the reaction rate. The generated growth structures are lumped in (1) directly linked, (2) aliphatically linked, and (3) pericondensed polycyclic hydrocarbons. It is found that the amount of aliphatically linked PAH increases with the amount of aliphatic content of the fuel mixture. Finally, a reaction scheme is presented displaying the most representative reaction pathways to form growth structures in each lumping class and their eventual interconversion. The present work—that applies a combined approach of macroscopic gas-phase kinetic calculations and atomistic reactive force field simulations—offers a good alternative to obtain structural differences of nascent soot for a broad range of thermodynamic conditions and detailed reaction mechanisms for the soot inception process.
A Theoretical Multiscale Approach to Study the
Initial Steps Involved in the Chemical Reactivity of
Soot Precursors


 !"#$%#& '()*&+
!, ((#-'. 
    /  0  1  2+    13    /            
//#"4 / 3 "  +/  "&
/"+1-+1
2+13/+3#25+0++"4
+/2+/2+-6##1#"   
2302++4 "+/3/#0/
-/+ 4 +  1 2+  13 7  2  1 0
3+/3 7/"326#
#+/4""3+/8
909/0:9//000142
 +2 /0 73 + 2/ 22 +-4
0 + //0" +/# /30 2+
"3    +/"    # #4   /3 ; 
//+1//2+/"&/+#2
5+;<"#1<221
"2+0+++2//4

,/1/2+
+/ +1 2  24
0  "  2+  #      
2  2  5  1    
+1"+"4,/
#13#=+/
+4//
/0 <0
  0  /  "    3  104
!/0+/#13
  1  +  +24        
>  2    /    "1  +
"4        1      
+ 2 # 2"4:  ,
/01"13+"
        1/  1  
/        3      +-"  
#"5<
#13/"
4%)
,  1      ?,@9    2  
  2  #  /  /0+
01 ? 79 -1" + + 2
#4"2/0+0
3,@+0<#13,@4A
  60  2    "    
 2 / "0 /
  2+  4   7"&
+    +/0  ?7*!9
  10  B  C   4  3    
  2  +/  4"4    +  
/2 2+-3
 +/ # "5 <
    2      /4D
+    "      0
  2    +/  1   /+ 
E+"10-
"    #      /  +/
4(;
    3         "0
>#024"4-
4 ,/ # "/
/ " #03 -
      + 4  0    
21102+ 
"+/3"
#04:  #      3    
1
+ +/2 /3
  /  =   /0        %
"    /  /      3
4)
"2 /+  ""
    /  2+    "&/  3  
/  /  "  +1    &
 /   /0 4    +
30  /  ++  2  +"  
//  #1 +=
2 3 7   F&F  3 
++=" 71" /0
+  104A;'  !-/+  #  3
    =  1  2    /  
>+ -11+1#. 3 +0
1  3  /  10  +"  /0
+=        /  /4
+-/+ # 2 -+
2  + """    >+  ""  
/0+=/04:
C F&F  2/0+0 1 "
"    /10  1    2+  2
+    >+  '%   #
1    ""    +=
++  2  /04);D  +  +=
61+  ('   //    /0
+=      B    C  2  
0/0+1
+/      #"  2  
/ "0 2 ? !,9 10 B  4
//5"4D
  /  "    /0  
#    -  +0  1  ##  
  /10 +=  B
  C  2      /=10  
      "  +  /4AD  
2/   "&/
"  +1    3 3#    
/  3  3      12    /
  2    /4:.  0
B43+ 7 1
3  /      +  0    2+
+4D:
, + 7 #  3 B  C
      2    #1
+= !#4#10
/0+=+
04:::        /  -/+
5"    +  1  2+  +
1"+1"2 7 
  /  3        
#10 10 " 2 
/4    2+  2  +  1  
+  +:%;:'   2
" +/  /  
-/+    //    5"4%.%
G//##
+    1  3      1  2+
13/34
#    #-/2
  +  1  2+  
+  +"        1
/2+4     +      
+/0    -/#    "  
2+1 2+13
" 7  "+1 2
##4              0
-/  2+      +
+4%    +  +"
// 0  +"3 "
7 1 & 
3  +  "3  2      +  
  3#/ 10 B 4%:%%
    //      /1    <
  2    /  1  
<  2  """    +  2
    /  + 1  +  
  /  24:(%)%A    
//    2 +"    /  
+1 / 1 #2
5  +  0+  ?*-94%D  
+ +" //  25
    1    13  +
1      +      +
"    3"  2  1  1"  
2+4 25/+=
6++3
        +  3
"  +/  H0    
  2  5  4  @"  
+"0    2+    1
#"    #  4  I"    4
+11 2+2+0
  +  /00    "  /
"  *-4%(%'  0  2    "  
/00  +    5  /0+=  
"          2+  "  
50  " /4 7
4++12%&+/
2    #"      /  /
" *-4).  0  "0  3  
2+23"8
2+25"2+
+    1""      50  
"/=  "4   +      4
+/0+2
+&= 7"*-10+"
+&+  +=    <
+/ /" +=2
/00    +0 1  7  
/  2    +/4))  0
3/2=2 7
  +/    3  /0  +=
  <  3  +&+  1
2+0#+/
/2 7=4) 20
0  #"        2+  2
+10  2 7 + 
4):  0  #    >  2
+=+12
++/0+
2    +=  /4   0  0
3    +  +      1
2+13 74)%J43
 + +- 2 + 7  +
  3    2    1  "
+1  /  "  +  "
2
*-    -/+  >+  +/4))
0 3 3 <+-2 7
 /0+=/13
7 23 < /304 0 10
2      3  +/    
2+  ++    "#  10  /0
213"
+/      /0    +
+4))
C"  "=        /
/308)A51"/3
"3  2    2+    2  +
+1"+=2
/+π-π-interactions 
  1"    #  1  2+  ?
  9  13    /
+4       /  0    
/30    #"    +  1
2+  13    /  +  
+          
< 2   /4   
+  /  //  2    
+"+0++/0
    +/      3  "  2  +1
4     #    "    "&/
#0+ +/  12+
+"+ <
+0+    "    2
+/6#  +/4
"&/+-//
+"04
      +    1      +
#253-
"  /4   !0  +  +  
5 3 7/0 
1,@2+"1
2+    &    /  
# +"3 +
"  *-4       -/        
+/  <        /
/    "3          
  2      3  <4   7  3
//    30    =    -/
13
"4332
  1  "  2    <  
+21
+1        2 /
-/<-1#4
"0
Phenomenological kinetics modeling4  
2  +1  3  +      
+"      +/+    
+ 23)D   / 
+/4  ,+ 3  2
.4/2.+//
  +1  "  +0+
42+-3#2+..
+K &    ..  +K      
/312.+K 
>2+/
2  2  "4
+++2*E
4)( 33+"#0
2  "      "    
7"3/4
Figure 1. 72"2//2*-+4?9/0?19
&/00    ?9  &/00    ?9  :&/00    ?9  &?&/0090  ?29  &?&
/0090  ?"9  &?%&/0090  ?9  0/LM/0  ?9  0/LM/  ?E9
>?90?91=LM/0?+91=LM/0?9/0?91=L"M/0?/9
?694
  ++  //  10  *E    4)( 3
#  "  2&  /+-  "
>+4 !/0 7/5 
++  1 +/#"
  ++4)()'      23  
/10&#1  ++    /  "
7  2+ #" +&"
"+-2++62A.;A
 # 30 #/ ++#
*EN  ++      1    /  
2+2" 74)'A:;A) 7
2+  2+  *EN  ++    /
3
#2++1#
#  -/+  4)()'A)AA  "
/#2"2
//"#+"*EN
++ " #02 
//4
 +/ "2+%.. ... 
/ 2 .."  / 
    +"    #  
++4/
  +    +  +/  2  2
+/  10  /2+"    /  /
" 5-+/  /
3"    2  +/  2    +/  2
2 +/  / /4   
6#
Φ
?+-/
"#  ,//" 9" 2+
4))3/2.4)4
    +/  "    6#
  3      #  +1
      3    /  4ADA(
 +/ /&/ -
//25"2
*- + 1  "3
,@4 -+/ +&
# +  + "3
5      0      +1  2
+    1        10
"+/+41E#
        /#    2  
2+2+  +  
/2+4   !-/+    3  
/0 +2  " / 0
  4:.     3
5/0+2
2      /4   +  &2&
/  0  1    *EN  ++
?  ,//+0    2    
9      7
/"/0+0
2 /0 +/4)(  72&
+/ 1 ... 
/      /0  +  2    
#  2  /0    "  /  
21+2
/4"/2
  "&/  +/  /    
/3  / 3 22
 +-++ 2  +22 /0 
3.4++4
,/0+20/0/
    //+  O    /"  +
+  1-  "  0    /0
+  3  +/  +    +
+4 0 +13
  +1  2  +  ?9        2
+  2  +  10  +"    
+/ #" . +4 ,
0+  1+    "  2  0  
+  0+  +4   2
3//45
  22 +-+++ 2
2  /0  -    //+&#      
H+12/
/40
++32+"
+22++
G+4 
 / 2/0  7 2  = 2
/0  3    1  3      
2+-/+4'.:%:.A' P 
2+&=/+
3103+"=
/  +/4  73#    
+/0"
++#"=&/
+/        *-  +
310"4
Figure 2. !-    /  +/  2    2  +-   6#    :4)  
+/2%.....4/&%...K&Q(.K
/2/0"?494
 +3 5"4
//2++0
  0  /      "  /
  -/+        
4
+  +    /0      
7/0/"/4*E
++  3        +
 "# 11
2 74   7  #
23112+
++ 13 7  #"  
0/  5# ++1
"<+4D.;D%
 1#&+ 
3"*-&/ " +-4  
 + 2 2   7 #
13  .    '..  //+  2      +
2 2  " / 3""
  +/  6#      2
+/4+=+/
# 7 + 2 3 ++=
/"12 7+
2+32
+12+*-+
1+0+1-3
+-++  2  ..  +  2    2  
<  +/4     -
+/2++
"#    5"  ,&,%    1  ,;,D   
,//+04
Reactive force "eld molecular dynamics4
  *-    3  /2+  "
  ,  G  +"  D) 3    B
+1    )..    3    &7#
+ +/"  2 .2
+/ 2.42+/0"25
/1  10  3    4DA    5-
#0 " +/3   
        2  +
  10  "    +/
+  3  "50  "  
  /4     5  //    6
++ /DD;(  3 0
10      4)   +  +  0+4
,+3+/?4"4...9
-  3  "  +  +  
3  +    //"  
+/4
0 2 + 1- 3
 /  /2. +
  +    3  /  
  /#  2    "
4(:72<
/+-#02+.4.:.4.)
"Q+:4 2G+3
+/# 10 " <  "
"+  2    4     <
  "+  44  <    +
      #      
   +    1  3     
++#023+
1""24
  0+  3  ++0  61
2../)..3#0<
10  /#"  1     "  13
+42612+/
1"+2*-3
3  2  1  1"    2+4   
+3242
  #0    3  +
3"- +-+++
2    "  +/  3      
"3&4   2    
2      /30  2    "3
1#"4
*$G
Impact of the combustion conditions on the
soot precursor pool.  ,"    1#
+  <  2  +-  6#
+/%%< 
3    2+  3    3  /1  
"( 7+-23"
      "    /
+/  2    G  4  " 
3 -+/ 12+
 + 2+124
  +/  #    "  2  
    3        
//+0  +4     +/  2  
6#        -  /
+-+/32++
#    #"  "  2  4)    )4.
" "50"+/R
S4)4A'(% 73#   2+ 
"  2    0  3  0    2&
+-?RT4)9+02+
4 7+/
  3  2  0    ?44    +9
6#4
"    3    //  +/  
- 2+ + 23
#   +++ ?%.. 9 
+-++?...9+/4
+/ 2    1
2      //+0  +4   +
" 23 1 +/
/0    0    /
" +     24  
%..  & 2+/ +
-#0  0  /0  3    ...  
/0    /&  2    1
+-0&/04
Figure 3. Rate of production of pyrene by the addition of smaller
aliphatic species to aromatic molecules (solid line) and the
reaction of equally-sized aromatic molecules (dashed line) as a
function of time (s). The contribution of these two distinct
pathways for the pyrene production are shown for three distinct
fuel mixtures and all temperatures at eq. ratio 3.5.
*&2&/  0  2  /0    *EN
++  3        "  
/2/03//308
92+2/0102
3"060&= +/ 
9    /3    2  +  ?/9
2"+      +  +4   
>2+/2+-
5
/"  1  2    3  /30
3/2/0  / 
"  :4         2    &2&
/&022+2/010
#+1" "#
//+0+4
" :0 3 %.. + 
/0  /    2+    /3
    2  60  =  +
/3?94 P02
/++/30
102//""54
"22>12
/300 +2%.. 
/302/02+ 10  2
3  +  /  +  +4   
/30  2  /0  2+  10    2
+/  +?9 "
"5+/1#%..3
  2+    "  +  2  2  +
/  /          "  /
      2  2"+  2    2
+      +/     *EN
++    "3  2  /  "  
/0110/32
0 74/3
0" & 1 & 7 & 
?79++30/2
2"34A();((  0 / 
-/+/
23"3++>
?"E94
The reactions involving small molecules (solid lines in Figure
3) gain in significance with increasing temperatures, owning
to an increased fuel consumption and faster fragmentation of
the fuel molecules with increasing temperatures. As a result,
the amount of small radicals found in the gas phase increases
and the pathways leading to pyrene and larger PAH by
addition of smaller radicals to aromatics is favored.
Therefore, at elevated temperatures also PAH other than
pyrene are more present. As the growth pathways of these
PAH in Raj’s mechanism imply the HACA mechanism, the
increased acetylene mole fraction resulting from the faster
fuel fragmentation at increased temperatures is the reason
for the higher diversity of the soot precursor pool
compositions with increasing temperature.
>22+-#0
2-//23+
4*EN++"32/0
 "+2+& 1
10    2"+  2  &    +
  23  10    +1    2+
+ /4   & 2 /0
"3"50<
2"+ / 0 2 
+  "4           2+  2
/02+23+/
*EN  ++      "5  +  2
02+/031"5
+/+40
 2 / " / 
  7&/30  3  34       
35":33"+
2       2  +-    /30 2
/0  2+  10    2  3  +
/""54
  +1  1#  ##  
/00//4C
+22  #03
" /3 +/?S(..9
3" +/3 +0
<-//
+/4+ 0
/0 3 " " +2 
  "  /  2  0    /  /
+/4
Common properties to all detected growth
structures.        2    *-
+   3 +
2"330=42
%%'  #  "3    3  1
2+  2 (
/+/4 10 2
"    3  /  "  
++22+-6#
    +/  ?"4  %94     /  
+-/++/1
+  / =4%A'(''. 
+  /      +  +  2  %.  "Q+
10++2+2:7(
3 //23 /00
4     /0  
6
Figure 4.   1  2    "
  "3  2+   /  2    2
+-6#+/4
+  /  /      23"  +
2+8  A7) U  :D  "Q+O  :.7A U  :DA  "Q+O
:7( U%. "Q+O :%7( U%A "Q+O :A7. U
%)"Q+4
+1/+/
/"+/+/
/        2  3  /0
+4    2  #  ).K  2    
  "    /0      
..K 2/0
/00      0  /0    
4      +/  
//-2+*EN++
+"/1 7/0
2+"&/4     
1/0"12
"//2 71
    1      1
/#4
")3+127&+/
/+22+1
2&+4 5"/23/
"  1    ?    +
+9 7Q  #"#"
.4)4"5/
+/1-/+37Q
.4)32/
=    -/+  >+  2    "
/4%A'(''.
  "    /    #"
13  #    "  + "4   
5"      3    7*!  0  10
43++12"
22" 2   /1
2"  "4'  +  (K  2  
+ "3
+1  2  &+  "          
"/ /     + 1
4    5"      3  
+"';'%  3    +
2+/0010 ,= 4%.
// "+ 3"
&+1  "    2      
/4    "    2  1#  10
,=    4  44    +  +
++ 3 " 7
3  D    +  "  3 /  10  
/  04  +  +    4
3        +  /  0  
/    13  +  +  
  /      2    
/4%22
3/ +
*- // %DK2 
"  -1    :AK  2    
    /0      :K  
04
Impact of combustion conditions on
structural di&erences4   +1 2 "3
     2  Figure 5.
+1 27&+#+12 &+
2  2    "    "3  2+
/22+-6#
    +/4        /
#27Q2.4)4
*- #" 2  3
"  2  3  /+    #0"    
/+ "A&4  
3 >2  /+
"3"4"4++=
2"+  4     -+/  "  A
/ "3#"
2    3  "  2  +/  
6#" /"
2&&Q2+-4
  1      +/  2  
/+/""2+
2      /+   160  
12+13 74
C    #"  +1  2    "3
+#"
2    2  +-    6#  
    "        2  
"3      3  "
+/        +  4
,    +  0+  +  
=    )..        +  1
1"+/"
2+    +  4     "
4"4/00 
23" +/   3
  "      2  
/  /    -/+  10
"  +  "34')'A  #  
/002+/+
    +  &1=  
/3  / 7 
"7&14  
3        1  ##    
3 74
  "/  2  /    /#  
1  "0  3    #0  
0  /04     /  +0
  2    <  13  
"    %..      A..  4   C  
+/1  %.. +
10/0 +/1 A..
 +#  #" +1
7
2  0  /0  4   G    
/  2    0        
 2  0
124
Figure 6. #" +1 2 + "3/ 2  *-    2 2 ?9 2
+/?19+/?96#4
Figure 7. */#  2  "/   3   <4 28
+  0    01  O  +8  +  /  /00  01
O"8+/0014
  "  /  2+    G  +
1 "=  +
1 2  + 4   5 
#/      3  0  
+  +4   ,  -1"  
2        23"  2    
+  0    01  
?G794     +      3  +
/ /  0   
3 &+ 13
+ +4    +1
  +    -1    /
/00 / + 2 +
+  /  /00  01
 ? 794    +/
+  -1"  3    +  +
+    10  /      
23"  2      +  /0
 01 ?794 */#2
    <  "/    /  
5"D4
#22
22+- 35" (4  C
   2  G7   7
              2  
7  4     2+  2  7
8
3/2
2+-45"2+4%
"0//+/2/
13  +  +      
/10/"2
2+/4
  "       1  5
"/2/4
    2  /      
222+-+/3
"'4+2+/002
,=    4  3      ++0
    0    4%.   
+42/
/"+/+04%
  0  2  "/      2
-/+0  &*  //0    1
/#0 /1
2  &7  2  "/4  @"    //
V+ 422+2+ /
+  2     "  /Q/:  ?44 
    /  &7  "/9    
2+2+ /24'D "00
  2      2+  2+  +-  2
/ +2# /Q/: 
    #  +        2  /
/  2      #    +
/ &7"/2+2+/
/  +/  3      1
5+0104'(
0#"+121+
21373/"
  0 1"  #" +12
1+2    24  
#"    2  1  +  /   
    1"    1  +  
/  "    .4:  2    
2+  10  /  &    .4  2  
2+10 / 4,2+
2++-2&"
-1"2WQ1"X1
  W/  "X  1      /
+/4
7            3    &*
 2 V+  4    4  J 
  +/  2      3    &*
"234&*
2/:?/91/1
/  "    +  10  +/"
#/" 2  U "
+      /  &7  "  +
+&7"+
0  &7  "  +    WX
1    1  P&7  "  +4''
73#          0  +"
            1
" 2++#1
&*
Figure 8.    #"  +1  2  "3  
  "  1  2+  2  
 +/ 6#
 2+- 3 < 2
/      2  2    2  +-4   
#  1  /    /#  4   
-/  30    +  2  #"  "3
  ?    "  '9      0
+
#"#/  " A  "# 
//+0+
4"0*+//0
    4  2    /  /"    /&
01=12+2+
3  /  0    "  
/  2    /0&0  .. 3
304
/013
2  +/      "3  
"  /    Q    
        /  +/
-2++1" +# 2
/&22+&2?24
"94 2"#0
"+27/"
  "    1  +    
1++/114  +
2 1 2+ 13 3 7 
    "3      + 0  
/      1    2  
  0  /    4   
1    13    ?/9
        2  +-    
+12"3"
/0      Q  
+/12
/  +    /    
"2/22+2
  "  /  "  +14   7  
2  / 7 
+2604
Analysis of the most common reaction
pathways.       1#  1
<2G7 77
#"+++
 /30   4
    -      +/        
   0/  #    ++  
44 / 2  "
2+/ 2+ 7
10"+/4/2+&7
&      2    00&
9
 2+  0 7  
2604   +    1=
3#13/0
  7   2+:' # 1
"12+13 74
73#    4  0      #0
2+/14
Figure 9.   #"  +1  2  
"3    2        
+/  6#      2
+-33/ 
22  2 +-4# 1
//#4
  G7        10    
2 7 7
   & 70 2310
  7&    0  10    7&
4     "  /  2  
++/30/5"(
3G7""4
+ 73
    /  2        1
1#4   73#    /30    2  
++4
@      #"    +
++0      /30  2
7      324    /30  
1027
2 7
2 7 72310"
/"    "      /  
13    3  +  4   ,  
/2 73/  
  +/      +    "
/"2G7+260
1#/
2 742
++27
2      %  "4     "  +
/ 2  3 /30 1# 
/5"(3 7
"""4
1 72##
 7 " "+ "
  "        /  2  
/  /4   *"  /"  
"+    #  1  3  
/0          "3    1=  
"/  7 24':.      1
" " "+ 2"3
/302 7/5"
(3  7 ""
14     //        2+  
G7     7   
/1  + 4   3  1#
"    +4       3  
26  3#  +/      
"    "  /"  "      7
54
Figure 10.   +  /  2    +
/#    /30  2    +
++0  2  4    /  2  
"/""4 
/  +/  0  23    
34B 2 -+/ /30 2
2<"/12
//"+4
  3    5"  %    "  2  +
+1""2+A%AD"Q+4
7  "3 "+ /
3  #      /    3  1#
"    +4     /  3
"10   2+/
/ 2+ 7  0 /
32+2
  "  /"4             
+&=      &1=
4   / >  "3 10
  7 "  "3 / 2
<  +4 "(
"=+++/304
+/
//  #1      +  0+
+  1  0    /  0  
23    2  1  +/  
5"  (4   2    "  /"    1
23  10    "        
  10    +1    #  #4
+3 27&+" /0
"5  12+ /4
*/ +"2 0"+  7
    "  /"    "+
    1  1#4     5"  
  3    5"  2    
10
4.  3  1#    +  +"
 "32 7 "
2+/4
,  #  2    +  +/  1#
  /30  2    <  
"/"+0++
/#,//"4

0102+
+"  +"0  3    
#"12+ 2/
  2  2  +0+    +
        2  2  
/4//1 7
 " 2++
  ++    #  2  5
+0+4
    1  10    +
+/  3  3    1  2+
-/+      #    
//  + 7 "3 10 +
12+4G
2  + " 10
  +/  "&/    
+0+3*-
 2  #0 " 7 
#"    "3  "  1  2+4
7+2 /  
## / 2   2 7 3
++//4
+ "#1
2       "3  1        1
#"  24  +  
"  - -0" /
#    0  1        
//4       +/  0
      +"0    1  
//3//+
2    -/+  >+      1
2+13 7/
1/4
+    "    3
+/      1    0
<  2          2
+/4       /
+  /  +  /    2  
/2/2"4
+122/
 /2/2
"//4C
+    4  /#  #  2  
/2/
/"+//0%
  +/  2    2  +/    
/  +        /
/    +    1  #"
-/+04
+ 260 1# /30
3 0= + ++
    "  2    1  #  "
"  4         3  2
7&+"212+
"    "&/"    "+
      /      
1=4
03+/1
2+ 13 7    /
++4   0  "      +
1 2+ 13 7 3 3 1 
/  /0  //  2    
/4  0    3  #0  2
    1  "  #"    "
"  2    2       1
2        0      +
#4%.   + 1 2+
13 7   #0 2 /
 + 1  +"2
  /      "3      3
1  /2 +/ "3 2
  /      1  #  10
/32+/ 74
ASSOCIATED CONTENT
,//"2+
G/26#0
/3002*EN++2
7/02-
/+/?C594
!-+//302G7?B
594
!-+//302 7?B
594
!-+//3027?B
594
AUTHOR INFORMATION
/"
!&+84&1Y2/42
+/33"12
4#"#//#
5#2+/4
ACKNOWLEDGMENT
/E#2"2+!/
@  7=  ..  *    #
/"+4Z"+AD))(2 G
/E4
ABBREVIATIONS
7 0+01
,@1
*P *2/
G7+001
7+/001
 7+//00
01
REFERENCES
?9P1[ Z4O P1[ !4O P1[ V4
-"08 !+"" G/ !##"2+
,2@5 4
2005(:;(:'4
11
?9  +  4&74O  1  !4O  1 ,4    #3    
+    +/  2  1  /  +4
2015:A;%:4
?:9V1 4 I4,&+ < 2"
2&212"+
+/4
2010G%.'4
?%944OG0,4V4O0G4C4O 4
4O4O G" 4 V4O  4 Z4O Z
,4O  \  4O   G4    "     2
1  1      +  0+8    5
+42013 ):(.;
)))4
?)9  4O  4 4O   C4
02#//2 2"""
"    //  &+-  +4   !"
#$%&2008'(A)A;AA:4
?A9 BC*44,8 G5
]5    +/4 #  %
2005%';%:A4
?D9  B  C  *4  4O  +  4  V4  ,
8 // 0 4
)"*2004+';%.4
?(9J4OBC*44O+P4O+
44+/ 2 2 2+   
  #0  2    4  )"  *2012
(:)'D;:A.A4
?'9BC*44OJ=4O4C4O+
G474OC"447*!,02
2+/54)2007D.;DD4
?.9J4O, 4O/4O "V4OB
C*4O+44O/4V4OG3 ,4+/2
  /    1  2"      /
+/"0   2+  ++&
1"  E"4  
,2014':;.(4
?9  B  C  *4  4O    4  V4  
#"  2  !<  2    P-0"  
  2  ,  2+    G&E  G
!"4,*"2006-':A%;:A'4
?9BC*44O+4V4,-8
G/  /    4 )"
*2003;'4
?:9  Z+  4  4O  B  C  *4  4O    ^4O
J=  4O  +++  4  *  2
1  -     #  "  1
""4)2016( )%);))A4
?%9  B  4  !<  2  +1&Z
/      +14 )"  #
%2016DA';DD)4
?)9  "0  V4  ,4O  B  V4  4O  ,5+  4  4
+18 Z#",=
+/  +/  7+ 74  
./,2000')A);A(4
?A9  //  V4O    74O    4  
+"22+3+0
/08 +/+- >+2  014
)"*2000-;:A4
?D9    4O    4O  1  4O  2 4
G  +"  2    2+      /0
 /" >3 4 )" *2002 -
:');%.'4
?(9  1  ,4O    4  ,4O  *E  4O  2  4O
I"74 *4O 1 ,4O + 4&P4 3 
 + 2  +1"4  )"
*2009+)A;A)4
?'9  4  4 ,=    4  4   2
,8  G0+ ,+ 2 0
G+=4)42002-(:.D;:%4
?.9I 4OJ"I4OI4OV 4B4O C"
74 =1 22/ 
+  /+-  0  >+8  <  2  >+
+/4)2005'%%;%%(4
?9 ," V4O  *4 44O2 4OC"74
+  +    #0  0  2
    /  =  1    +
/+-0>+4)"*2006 
D;D4
?9C"74 +2   
&/+>+4)
2011%;AD4
?:9    V4  74  +  -+8  #  2
/0  +  01    
>+4)2005':(;:((4
?%9  7+  V4  G4O    V4  74  +
/    2  /0  +
01  4     )  2008 -
A%';A)A4
?)9,1174O4O//,4V4O,+
4 *4O *3 4 *4 !-/"  * 2 7  
+2,8 0G+=4)
02010'A;'AD4
?A9    4  ,4O  6  4  V4O  2  4  
6#  0  2    "  2  /00
+  01    "  +/4 
))2012%.(;%.'%4
?D9 !# 4O B 4 +0+ 2 /0&
+  01  "      <  2
1  /  4   )  2013
(:D;(%:4
?(9    4  ,4O  6  4  V4O  2 4    
/ G#/+ 2 Z /
 2 00 + 7014 
)%)2010+A(:;A')4
?'9    4  ,4O  6  4  V4O  2  4  
21    +  2  +
13/00+014
)02011')%;A.4
?:.94O^4OG"- 4!#2
+8B12,02,
/ ")" /,
20157"04
?:9 " ,4&74O B 4 &+
3  /      0  +  2  
2+014 )
2011A':;D..4
?:9!#44OG3,44O+4V4
+/2 #10+"  
/4 )2015
D(D;D'%4
?::9  !#  4  4O  G3  ,4  4O  +  4  V4
"# 1 2 7 +
10  #1  "    
4)2017+':);'%)4
?:%9 "0 4 *4O Z4 4O  ,4 !4
*# /00+ 01 +=
#4))2018
-'.'A;.':(4
?:)9  "0  4  *4O  !#  4  4O  B  4O
+4V4 2# 7+=
"#104  )
2018, , ..:;.4
?:A9!#4O G+B44OB4P-0"#
2+4)2017+();(:4
?:D9 V 4 P4O G+ 4O !# 4O
+/14 4O , 4 !4O /&BG44O
*&7  4  4O  C  4  *4O  B  4O
  74  4  *;    /0
/2++14
)2017+D'';(.A4
?:(93V4,4OV4J4C4O!# 4OB43
/#+ 2/00+ 01
12
+=  //04   )  2015 
(D;(:4
?:'9V 4P4O7&Z44O,
4 !4O C 4 *4O  74 4 *&
1=  01&      +0
-/ / "34#2018+
''D4
?%.9,=4O++4O4OZ4O
 4O 0Z4OG_ 4O Z4 "
  /    2+  10  +  2
+/04)2018, (();('4
?%9  +  4  G4O  ,  ,4  4O  +  4O
74G2/0"&
00 + 701  ," +
3  +/&,+/"  7"&*  +
,/+04)2018--'::(;
':%'4
?%9V4O 4O "04*4O+4
V4O  C"  74    =  2  "
  2    "      2    >3
<  >+4   )  )  2014 +
)(A;)(D)4
?%:9  G`  4O  B  4      +  2  
2+  2  +  01    /+-
+>+4)1998-%);%::4
?%%9  B 4O  GN  4O  GN  4  "  2
/  2+    +1    /004
),#1999:%::;:%%4
?%)9  G`  4O  B  4  G  "  2  
 Z3   +  /
+-+4,*"2005(D';(A4
?%A9    V4  J4  C4  ,  ,+  2
1 /  + 
+14G@#02"20144
?%D9  #  G    4 4O  G"/  ,4O   4O
Z  C4  4  *-8a    *#      2
7014)2001'':'A;'%.'4
?%(9I"4OI"4OJ4O^^4+""
12+100/003+
#  2  5  +4   )  )
2015%A';%(.4
?%'9^^4O"4O  J4O I" 4 
*#  &  ,+      1
# 2+ 04  ) )
2017-D).;D):4
?).97,4O^4O4OI"4O^4OZ4
*#"      +0  2  ,  /
+10*- G0+,+4
,*"2017(%:%;(%%%4
?)9]4O# G444O 474+
2  /    /  2+  /00  +
018    *-  +  0+  04
)2017-:(.;:((4
?)9]4O*J4O474O#G 4 4
G0+      2  #1  +&+
+=  2  /00  +  014 
)2017%%:.)4
?):9]4O7G4O474OJ^4G+=
2 00 + 701  
*    +  4     )  
2018--(D.;(D.(4
?)%9    ]4O    4  74    +  
/00  +  01  2"+
"34)2019
.:;.:.4
?))9  J  74O  " C4O   4O   G4  ,0  
   "3 2+ 7  +
#  /    >+  +/  10  *-
+ 0+4 ) , #2018 (D%(;
D)D4
?)A9C"74 +2   
&/+>+4)
2011%;AD4
?)D9 )/1234O  *  G"8  ,  G"
.:4
?)(9*E4O 4G44O+44O",474
 ++ 2 " "2 2
"  /00  +  014 )"
*2012()..;))4
?)'9  ,4O C"J4O ",4 74O ,0,4 4
+/ <  7   2+ 
>3<>+2""24
)"*2017 %A;A.4
?A.9  G3  ,4  4O  I"  ]4O  +  4  V4O
,#0  4  4O  *  @4  //  2  
 7 "3 +   2+ 
+  >3  0Q  < >+4 )"
*2011 A(;A')4
?A94O 4O 4!4 3 "
 +" 0 2 7 2+ 2+ 6
/  2  +14 )"  *
2016).;AD4
?A9  J4&=4O " ,4&/4O  J4&64O ] V4O  ^4O
I 74 -/+0 2 /00 +
01      +  2+   ZG  "
2  3  ++  "4 )  #""  (-
2016+A.;D4
?A:9,4OC"74O24OC"I4O4O
+4O I"J4O 4O J"V4O ,0,44
00 + 01  /00 2
"  "  ?&/Q&Q94
)2019'':;..4
?A%9*E4O*4V4O",474O,0,4
4 7 Z3  10 /"0 8
+  G#/+    +/ 4
)2014 (A);(()4
?A)9 C" J4O *E 4O" ,4 74 7 "3
++ 0"<  72+
>3  <  >+4 )"  *2013
+'AAD;ADA4
?AA9 7 I4O I 4O ^ 4O C4 #" &
#  #5  6"      )"78  *
.'4()b.8..4
?AD9  ++  4O    4&4  7"  +1
+/2  * 2  G
!"O,!'((4
?A(9 "4O, 74O7V469":"&
$  6;  "$,  "$  8:",
$$<&D%4OBG&
8Gc2'((4
?A'9G"- 4O4O^4O
4O1"G4O!4+/#0
2 2+ /  WX 
W"X  3  /  /+-  +  >+4
)"*2017 ):;AA4
?D.9P4O3J4O G34OC14O
 4O,J4OJ4O 74O!4O
4+++22+"&
+&3"  /00  +  014
)"*2015+-AD.;AD(4
?D9,4O,4O04O4,
G  ,+/"    ,/+  ,0  
+ 2 00 + 701 
 004  ) 2007  (:.(;
(:%4
?D9"0 ,44OC4V4 52#&
 ,#&*" 00 + 701 2+
 ,/ 00 2 &G4 ,  *"
2011-%)D;%)D4
?D:9  V  I4O  7"  74O  I  C4O  ]  4O  I"  4
!-/+    "  #" 2  &G
13
00,/ 4,*"2014
- A.';A.(4
?D%9  "  4O  G+  P4O  ,"  4O
70  V4&4  0 2  /00  /  2+ "
01      +"  2  "3  2
/00  +  01  3  
+042009 +%(;A.4
?D)9 BZ4O /44O !4 V4O
Z4O#Z1" ,4 V4 4O,EV4
Z4O  I"  4  +0  3  G4   )" )
2001--':;'AD4
?DA934O# G O ZC4
4:4*-#252+0+
+201-4   )
2008-.%.;.):4
?DD9V 74O ^ 4O  74O ^4O  V4 +
#"2 "5 / 3
3    *-  +  0+  +4   
$,,2018.):;.)%4
?D(9    I4O  ,  C4O  I  4  +1
+     2    #8   *-
,+4 ,*"2018 - ();
(A:4
?D'9V74OCJ4OZ4O,^40+
#"    0"  /  10  /00
+  01  "5    /
34$,,2016:(:D;:(%:4
?(.9&4O+44O*V444O
# G 4 44O 3 V4 4 +1 2 
  4  A      +  "   +
/*-#254
)"*2012(D;()4
?(9I" 4O^4OV4OZ4 +
* ,+ 2  00 # *-
  G0+4 ,  *"2013 -  '%;
')4
?(9I"4O^4OV4OC"I4OZ"^4OZ
4O ," C4 002 ,+ 10 Z @&
  *-  G  3  +2+  04
,*"2014- );):%4
?(:91V4G4O,1G44!<2ZG0
 B/=    G/ 2
G,/0O,!''A4
?(%94O,,44!<26#
  /          /+-  &
/>+4)"*2002A;(4
?()9*74O73V44 + 2 /00
+  01      "3    d
#3  2  +    /304 ,  ,
)"#2000-+)A);A.(4
?(A9    4O  C"  74 G  +"  2
 /   "34  ) 
1991-))';)AA4
?(D9    4O  0  G4  C4O  Z  C4  4O
,,4!4G+"2 2+
 &1 /00 204  )
1985-'((D;'.4
?((9  C"  74O    4    2  *
H2 +0 # * 2
0 3 B0  +*4 
)1994( %A);%('4
?('9+4O4O^4OP"4&
4O  4O  !4O G"- 4O  4
@#"     "38 C
0    +  /+0  +  
04)2019, =();(:.4
?'.9/4O/4O2e4O1*4O
" 4O  4O E4/+
02" 72&0>+4
)2007)%D;)):4
?'9    4O  ,"  J4O  0  V4O    V4O
G0V4OJ" C4 4O 2 4  2
 /   < >+4 )2019 =
A:);A%4
?'9  V  4  P4O  G+  4O    4O  !
Z10 4O +/1 4 4O , 4 !4O /&
BG4 4O *&74 4OC4 *4O
B 4   +  + 2 " 2
-0"012+>+4  2
$#>#2016(:D%;(:D'4
?':9  C  *4O  G+  G4O  ,+&  *4O
 C4 4 V*O 4 Z/0 "3
+08 5#&    -&++1  "  >/  4 
)2008-);:.4
?'%9  C  *4O    4  G  
    +  2  "/&"  "34  
)2010A(';D.:4
?')9  GN  4  +1&2+  /4
)2009-)':;A:4
?'A9+ 4O  J4O "4O, ,4O
G4 4 + 2 ,+ *G
+1  10  *"  +/4 #
%2001'A%(;AA4
?'D9V+44OZ+44OBC*44O
7"4&74O#V4G4O"0V4,4,-
  /= 4 )" *
2014+');'A)4
?'(9 J4O,"4O#Z4O ^4OC"4O]
J4O  J"  ^4  ,2  2  "/    /:Q/
01=    2  &0    2+    
"  2  3  &/   &/Q4
)#""(-2016(.(;:4
?''9C" 4O ,"4O," V4O # Z4O " 74O
I"  C4  /  ;7    -0"  2
2  "/  2    &0  8
=    +/      -
1#4)2013:.'';:.A4
?..944O2+-4O" 4&!4O 
4  G  1#  2  /     
/  /   3&/  /+- 0
>+  #    *+  //04   )
2019(A';(DA4
?.9 *E4O 44C4O4,4O ,4O
,0  *4  4O  2  4  3  /00  +
01 ? 79 2 /  +/# 
+  /  2    +/  2  +1&
" 74)2010:';::4
?.9    4O  0  4  C4O  3  4  V4
70"  +"    /0+  "34 
)1998-A));AA4
14
... Although the thermodynamic data of many PAHs are still subject to uncertainty, the reversibility of a nucleation process hypothetically driven by the dimerization of small PAHs has been used to successfully reproduce soot volume fraction profiles 9,10 and NSP size distributions 11 in several laboratory flames. To explain the dimerization of PAHs at high temperature, the formation of covalent carbon-carbon bonds between PAH monomers [12][13][14][15][16][17][18] , or alternatively rapid radical-driven clustering reactions 19,20 , has been invoked. The hydrogen abstraction acetylene addition (HACA) mechanism, the defining feature of which is the kinetic-thermodynamic coupling, has also been recently generalized by postulating the formation of doubly bonded covalent bridges between PAHs 18 . ...
... The molecular formulae are consistent with benzenoid PAHs (C 14 H 10 + , C 16 H 10 + , and C 18 H 12 + ), PAHs containing 5-member rings (C 12 H 8 + and C 16 H 10 + ), radical ions obtained from hydrogen elimination reactions from PAHs containing a fivemember ring (C 13 H 9 + , C 15 H 9 + , and C 17 H 11 + ), and sidesubstituted PAHs (C 16 H 12 + ). These molecular formulae are also consistent with the structural formulae proposed in soot formation models based on the dimerization of low-mass PAHs 13,[15][16][17][18][19] . The many correspondences between sums of pairs of m/z in mode 1 with individual m/z in mode 2 suggest that the dimerization of C12-C20 hydrocarbons having individual [H] 0.35-0.43 ...
... However, since dispersion forces are too weak to overcome the entropy penalty of dimerization above 1000 K 7 , van der Waals dimers and dimer radicals cannot survive at flame temperature, unless covalent C-C bonds form to stabilize them. Accordingly, many of the identified molecular formulae are consistent with the structural formulae expected for covalently stabilized homo-and heterodimers formed from monomers in mode 1 13,17 , and with many structures observed by atomic force microscopy 14,35 . ...
Article
Full-text available
The role of polycyclic aromatic hydrocarbons (PAHs) in the formation of nascent soot particles in flames is well established and yet the detailed mechanisms are still not fully understood. Here we provide experimental evidence of the occurrence of dimerization of PAHs in the gas phase before soot formation in a laminar diffusion methane flame, supporting the hypothesis of stabilization of dimers through the formation of covalent bonds. The main findings of this work derive from the comparative chemical analysis of samples extracted from the gas to soot transition region of a laminar diffusion methane flame, and highlight two different groups of hydrocarbons that coexist in the same mass range, but show distinctly different behavior when processed with statistical analysis. In particular, the identified hydrocarbons are small-to-moderate size PAHs (first group) and their homo- and heterodimers stabilized by the formation of covalent bonds (second group).
... However, a reactive molecular dynamics study of colliding aryl, aryl PAH found collision efficiencies one to two orders of magnitude lower than those required for nanoparticle formation as discussed [388]. Some other reactive molecular dynamics studies have suggested rapid clustering of PAH radicals with close to unity C E [472,473]. However, the high densities of soot precursor molecules used, 0.2 g/cm 3 , significantly enhanced the clustering, overshadowing the importance of fragmentation in flames where PAH densities are closer to 10 −6 g/cm 3 [115]. ...
... Furthermore, high temperatures (2000-2500 K) have often been used to speed up these simulations, leading to direct hydrogen loss from PAH edges. This does not occur, at any significant rate, at flame temperatures without a radical abstraction, which means that the number of radicals in these simulations is much higher than in the flame itself [472,473]. Mao et al. demonstrated the importance of temperature in reactive molecular dynamics [474]. No clustering was seen at 2000 K for PCAH such as coronene, however increasing the temperature to 2500 K led to hydrogen loss and chemical inception [474]. ...
Article
Full-text available
The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.
... These molecules have been considered by researchers as being representatives for all the PAHs, but the extension of the list was suggested several times (Venturini et al., 2018;Inceog˘lu et al., 2019;Li et al., 2019). In addition, PAHs are considered as the precursors of soot (Keller et al., 2019), which initiated several combustion chemistry studies to build a model to predict their formation (Slavinskaya et al., 2012;Appel et al., 2000;Raj, 2009;Wang et al., 2007;Frenklach and Wang, 1991). Numerous growth mechanisms have been already proposed and a significant amount of data of elementary reactions were produced (Narayanaswamy et al., 2014;Zhu et al., 2017;Wang et al., 2013;Lin et al., 2016;Liu et al., 2016;Mao et al., 2017;Yuan et al., 2019;Liu et al., 2018;Goldaniga et al., 2000;Whitesides and Frenklach, 2010). ...
Article
Full-text available
Reaction initiation points of the 16 priority polycyclic aromatic hydrocarbons (PAHs) have been determined by calculating all the different C-H bond dissociation enthalpy (BDE) values. Six density functional theory methods (B3LYP, B3LYP-D3, B97D3, M06-LD3, M06-2X-D3, and ωB97X-D) in combination with 4 basis sets (6-31G(d), 6-31+G(d,p), 6-311++G(d,p), def2-TZVP) have been applied and the most feasible combination has been selected. The BDE values and the corresponding bond lengths have been used to determine potential attack points on the structures. The studied molecules have been categorized structurally as well, within which the position of the hydrogen atoms is considered. Results show that most of the hydrogens are in zig-zag positions, and the BDE and bond length values for the 16 priority PAHs are in a range between 342.0 – 485.6 kJ/mol and 1.0817 – 1.952 Å, respectively. Most of the initiation points are represented by armchair and peak hydrogens. The lowest and highest BDE and shortest and longest bond length values belong to fluorene and acenaphthylene where the hydrogens were aliphatic and in peak position, respectively.
Article
The aggregation of coronene is relevant to understand the formation of carbon nanomaterials, including graphene quantum dots (GQDs) that show exceptional photophysical properties. This article evaluates the influence of carboxyl and amine substituting groups on the aggregation of coronene by performing a global optimization study based on a new potential energy surface. The structures of clusters with substituted coronene are similar to those formed by un-substituted monomers, that is, stacked (non-stacked) motifs are favoured for small-size (large-size) clusters. Nonetheless, the presence of carboxyl and amine groups leads to an increase of the number of local minima of comparable energy. The clusters with substituted monomers have also shown to enhance the attractive component interaction, which can be attributed to weak induction and charge transfer effects and to stronger electrostatic contributions. Moreover, the calculated height of magic-number structures of the clusters in this work is compatible with the morphology of the GQDs reported in literature.
Article
This study investigates the soot formation process of diesel combustion using molecular dynamics simulations with reactive force fields, to examine the effects of five oxygenate additives on diesel soot reduction. The newly improved ReaxFF CHO2016 parameters are employed due to the feasibility on modelling the kinetics of large hydrocarbon fuels and describing the chemistry of carbon condensed phases. The detailed pathways of soot formation examined include the thermal decomposition of fuels, formation of aromatic rings, the mechanism of nucleation and mass growth of nascent soot. The morphological developments of the soot inception are obtained, together with quantities indicating the physical and chemical properties, such as mass, size, C:H ratio and aliphatic-to-aromatic C:H ratio. The role of aliphatic-substituted aromatics on nascent soot coalescence and the appearance of aliphatic side chains in soot core-shell structures are identified. The concentrations of C-atoms in nascent soot particles contributed from oxygenates blending with diesel are quantified and visualized for the first time, leading to identification of the relative sooting propensities. The mechanisms of molecular structure of the oxygenated additives, i.e. the existence of ester, alcohol, carbonyl group and ether, on soot precursor mitigation are elucidated via evaluating the early formation of CO and CO2 quantitatively during the thermal decomposition.
Article
The irreversible dimerization of polycyclic aromatic hydrocarbons (PAHs) – typically pyrene (C 16 H 10 ) dimerization – is widely used in combustion chemistry models to describe the soot particle inception step. This paper concerns itself with the detection and identification of dimers of flame-synthesized PAH radicals and closed-shell molecules and an experimental assessment of the role of these PAH dimers for the nucleation of soot. To this end, flame-generated species were extracted from an inverse co-flow flame of ethylene at atmospheric pressure and immediately diluted with excess nitrogen before the mixture was analyzed using flame-sampling tandem mass spectrometry with collision-induced fragmentation. Signal at m/z = 404.157 (C 32 H 20 ) and m/z = 452.157 (C 36 H 20 ) were detected and identified as dimers of closed-shell C 16 H 10 and C 18 H 10 monomers, respectively. A complex between a C 13 H 9 radical and a C 24 H 12 closed-shell PAH was observed at m/z = 465.164 (C 37 H 21 ). However, a rigorous analysis of the flame-sampled mass spectra as a function of the dilution ratio, defined as the ratio of the flow rates of the diluent nitrogen to the sampled gases, indicates that the observed dimers are not flame-born, but are produced in the sampling line. In agreement with theoretical considerations, this paper provides experimental evidence that pyrene dimers cannot be a key intermediate in particle inception at elevated flame temperatures.
Article
Full-text available
This work presents a dynamic and kinetic study on the dimerization of polycyclic aromatic hydrocarbon (PAH) molecules and radicals under flame conditions using reactive force field (ReaxFF) molecular dynamics (MD) simulations. The accuracy of the ReaxFF force field is evaluated through comparing with quantum chemistry (QC) calculations of the barrier heights and species concentrations of PAHs reacting with H and OH radicals. A series of homo binary collisions between PAH molecules/radicals are performed to reveal the influence of temperature, molecular size, PAH composition as well as the number of radical sites on the dynamics and kinetics of PAH dimerization. Instead of directly forming the strong covalent bonds, the majority of the binary collisions between PAH radicals are bound with weak intermolecular interactions. Effects of oxygen on PAH radical dimerization are also investigated, which indicates that the oxygenated PAH radicals are less likely to contribute to soot nucleation. In addition, temperature, PAH characteristics and radical site dependent collision efficiency for PAH radical-radical combinations is extracted from this study.
Article
Full-text available
The transformations from polycyclic aromatic hydrocarbons (PAHs) to polymers and finally to young soot were simulated under different initial conditions at flame temperatures using Reax-FF MD. At low temperatures, PAH heterogeneous nucleation is mainly through physical interactions, but these clusters are not stable at higher temperatures. The molecular dynamics simulation results at 1450 K suggested that soot growth at intermediate temperature and low radical concentration conditions mainly controlled by chemical reactions involving carbon addition directly, without hydrogen abstraction. Some branched chains can be added to the PAH edges and small PAHs grow into dimers, trimers, or larger molecules through combination with unsaturated aliphatic hydrocarbons. At higher temperatures, radical-molecule reactions, such as via the HACA mechanism, reactions of aryl radicals and aryl-acetylene, and radical-radical reactions involving the combination of two aryl radicals, are dominant in soot formation because of high radical concentrations.
Article
Full-text available
The evolution of the internal structure of soot particles was studied in a coflow diffusion flame. Soot particles from the flame were imaged using high resolution transmission electron microscopy. An algorithm to quantify the nano-structure of the particles was extended to study the radial distribution of fringes within the particles. The approximate size of the molecules in the particles was calculated from the fringe lengths, assuming planar peri-condensed PAHs. The molecules are slightly larger (∼16 rings) and more stacked at the core than at the surface (∼12 rings) of the youngest particles sampled, suggesting that the particles could be formed via the stabilisation of a nuclei of larger PAHs and condensation of smaller PAHs on their surface. In the lower-temperature region of the flame the molecules grow mainly at the surface of the particles, whereas the molecules in the core of the particles become less stacked and slightly smaller, indicating some degree of nano-structural mobility. In the higher-temperature region of the flame, a graphitisation process takes place, with the development of a shell of longer (∼20 rings), flatter and more compact molecules, and an immobilised amorphous core. At the tip of the flame the particles are oxidised, mainly through surface oxidation.
Article
A reaction mechanism having molecular growth up to benzene for hydrocarbon fuels with up to four carbon-atoms was extended to include the formation and growth of polycyclic aromatic hydrocarbons (PAHs) up to coronene (C24H12). The new mechanism was tested for ethylene premixed flames at low (20 torr) and atmospheric pressures by comparing experimentally observed species concentrations with those of the computed ones for small chemical species and PAHs. As compared to several existing mechanisms in the literature, the newly developed mechanism showed an appreciable improvement in the predicted profiles of PAHs. The new mechanism was also used to simulate PAH formation in counterflow diffusion flames of ethylene to study the effects of mixing propane and benzene in the fuel stream. In the ethylene–propane flames, existing experimental results showed a synergistic effect in PAH concentrations, i.e. PAH concentrations first increased and then decreased with increasing propane mixing. This PAH behavior was successfully captured by the new mechanism. The synergistic effect was predicted to be more pronounced for larger PAH molecules as compared to the smaller ones, which is in agreement with experimental observations. In the experimental study in which the fuel stream of ethylene–propane flames was doped with benzene, a synergistic effect was mitigated for benzene, but was observed for large PAHs. This effect was also predicted in the computed PAH profiles for these flames. To explain these responses of PAHs in the flames of mixture fuels, a pathway analysis has been conducted, which show that several resonantly stabilized species as well as C4H4 and H atom contribute to the enhanced synergistic behaviors of larger PAHs as compared to the small ones in the flames of mixture fuels.
Article
At the present day, ultrafine soot particles have become the object of increasing attention due to their well-documented adverse effects on human health and climate. In particular, understanding soot nucleation is one of the most challenging problems toward a more controlled and cleaner combustion. Detailed information on the chemistry of nascent soot particles (NSPs) is expected to provide clues on the soot formation and growth reaction pathways. Herein, the early steps of soot formation in flames are addressed by investigating the chemical composition of NSPs and their molecular precursors by secondary ion mass spectrometry. The originality of this work lies on the combination of several factors that include: two different approaches to gain access to soot nucleation in premixed flames (analysis of a nucleation flame in which NSPs nucleate all along the reaction coordinate without growing in size) and in diffusion flames (separation of the precursors and soot regions); a sampling procedure that allows a rough separation of the condensable gas phase from soot particles; a large database of collected samples and high mass resolution that enable the use of an improved data reduction protocol based on hierarchical cluster analysis and principal component analysis for the data mining and interpretation.
Article
This paper provides experimental evidence for the chemical structures of aliphatically substituted and bridged polycyclic aromatic hydrocarbon (PAH) species in gas-physe combustion environments. The identification of these single- and multicore aromatic species, which have been hypothesized to be important in PAH growth and soot nucleation, was made possible through a combination of sampling gaseous constituents from an atmospheric pressure inverse coflow diffusion flame of ethylene and high-resolution tandem mass spectrometry (MS-MS). In these experiments, the flame-sampled components were ionized using a continuous VUV lamp at 10.0 eV and the ions were subsequently fragmented through collisions with Ar atoms in a collision-induced dissociation (CID) process. The resulting fragment ions, which were separated using a reflectron time-of-flight mass spectrometer, were used to extract structural information about the sampled aromatic compounds. The high-resolution mass spectra revealed the presence of alkylated single-core aromatic compounds and the fragment ions that were observed correspond to the loss of saturated and unsaturated units containing up to a total of 6 carbon atoms. Furthermore, the aromatic structures that form the foundational building blocks of the larger PAHs were identified to be smaller single-ring and pericondensed aromatic species with repetitive structural features. For demonstrative purposes, details are provided for the CID of molecular ions at masses 202 and 434. Insights into the role of the aliphatically substituted and bridged aromatics in the reaction network of PAH growth chemistry were obtained from spatially resolved measurements of the flame. The experimental results are consistent with a growth mechanism in which alkylated aromatics are oxidized to form pericondensed ring structures or react and recombine with other aromatics to form larger, potentially three-dimensional, aliphatically bridged multicore aromatic hydrocarbons.
Article
Studies on the coal gasification process in supercritical water (SCW) were carried out with the ReaxFF molecular dynamics (MD) method, in which the Wiser model of the coal molecule was adopted. The results show that hydrogen production increases with increase of temperature and water–coal mass ratio. It is also found that the coal molecule breaks into small fragments before it reacts with water molecules. The detailed chemical reactions and pathways of hydrogen generation during the gasification process are disclosed. H ions are found to be the main source of hydrogen generation, and C–H–O compounds or radicals are the most essential reactants throughout the reactions producing H2 and H ions. OH ions can significantly accelerate the oxidization of organic fragments to produce C–H–O compounds and radicals, which explains how catalysts of alkali salts such as NaOH and KOH improve hydrogen production.
Article
Raman spectra of soot particles were monitored in the gaseous flow extracted from the burning regions of two low-pressure premixed ethylene flames, for the first time. The flame conditions were chosen to explore the diversity of soot nanostructure in slightly sooting flames. Evaluation of the Raman spectral parameters revealed that the soot particles exhibited a strongly disordered structure and a large proportion of sp hybridization of the carbon. The appearance of sp carbon chains composing up to 30% of the total carbon content as well as an olefinic component may indicate their important role in soot nucleation and growth in low pressure ethylene flames. Hence, Raman spectroscopy of soot particles in the aerosol phase revealed that accretion and cyclization of the aliphatic carbon including sp carbon chains could thus be of importance for the initial soot growth and require more investigation.
Article
Fuel additives were widely used as the octane number improvers, oxygenates, emission depressors, corrosion inhibitors to promote combustion processes of liquid fuel. In this work, six kinds of fuel additives including ethanol, butanol, DMC, DBC, MTBE, and TTAG were studied by ReaxFF MD simulations. The bond dissociation reactions were found to be more dominant at early stage than oxidation reactions, which means the unimolecular reactions was the main pathways of primary reactions in hydrocarbons combustion. The rate constants of primary reactions of ethanol combustion were much smaller than other systems, which were in good agreement with the product distribution analysis and previous work. The main reaction pathway and relative rate constants for all systems were evaluated. Four kinds of main radicals including •CH3, ••CH2, •OH, and •HO2 were detected and the number variation with time were presented. The number of •OH radical was the largest among those four radicals and it was found to gradually increase with time except for ether systems, and the number of •CH3 and ••CH2 sharply increased first and then gradually decreased. Hopefully, the results obtained in this work would be helpful to future design and screening of new fuel additives.
Article
Mystery surrounds the transition from gas-phase hydrocarbon precursors to terrestrial soot and interstellar dust, which are carbonaceous particles formed under similar conditions. Although polycyclic aromatic hydrocarbons (PAHs) are known precursors to high-temperature carbonaceous-particle formation, the molecular pathways that initiate particle formation are unknown. We present experimental and theoretical evidence for rapid molecular clustering–reaction pathways involving radicals with extended conjugation. These radicals react with other hydrocarbon species to form covalently bound complexes that promote further growth and clustering by regenerating resonance-stabilized radicals through low-barrier hydrogen-abstraction and hydrogen-ejection reactions. Such radical–chain reaction pathways may lead to covalently bound clusters of PAHs and other hydrocarbons that would otherwise be too small to condense at high temperatures, thus providing the key mechanistic steps for rapid particle formation and surface growth by hydrocarbon chemisorption.