Ion-Molecule Reactions in Flames

17 Ion-Molecule Reactions in Flames H. F.

CALCOTE

and

D.

E.

JENSEN

Downloaded by UNIV OF ARIZONA on November 21, 2012 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1966-0058.ch017

AeroChem Research Laboratories, Inc., Princeton, N. J. 08540

Low energy ion-molecule reactions have been studied in flames at temperatures between 1000° and 4000°K. and pressures of 1 to 760 torr. Reactions of ions derived from hydrocarbons have been most widely investigated, and mechanisms developed account for most of the ions observed mass spectrometrically. Rate constants of many of the reactions can be determined. Emphasis is on the use of flames as media in which reaction rate coefficients can be measured. Flames provide environ ments in which reactions of such species as metallic and halide additive ions may also be studied; many interpretations of these studies, however, are at present speculative. Brief indi cations of the production, recombination, and diffusion of ions in flames are also provided.

TT\espite the widening theoretical and experimental interest i n phe• ^ n o m e n a associated with ionization in gases which has been prompted in recent years b y practical problems, few reliable rate constants for ionmolecule reactions at low energies are available. A flame provides an environment particularly suited to study such reactions since its pressure, temperature, and composition are easily controlled, and the average energies associated with individual molecules (a molecule at 2 0 0 0 ° K . has an average translational energy of 0.26 e.v.) are low b y general standards of mass spectrometry. Furthermore, the comparative simplicity of charged and uncharged species found in flames (some typical equilibrium flame compositions are given i n Table I) makes it possible to interpret observed reaction rates in terms of fundamental parameters and to base estimates of unknown rate constants on more than empirical grounds. Flame temperatures vary typically from about 1000° to more than 4000° Κ ; where possible, we refer our discussion of reaction rates to a temperature of 2000°K. T h e proportion of ions present i n laboratory flames without metallic additives under such conditions may be between one part in 10 and one part in 10 . 13

5

Most information concerning ion-molecule reactions in flames has been obtained from mass spectrometric measurements, but some i n ferences have been drawn from results of other types of experiments 291

In Ion-Molecule Reactions in the Gas Phase; Ausloos, P.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

292

I O N - M O L E C U L E REACTIONS IN T H E GAS P H A S E

Table I.

H /N /0 ; (N :0 = 3:1) 2

Flame

2

2

Equivalence Ratio, φ Pressure, torr Tempera ture, °K. [N ] [H 0] [H ] [H] [OH] [0] [0 ] [CO] [C0 ] [NO] [CHO]

2

2

2

2

— — —

2

10" 10" 10" 10~

6

8.4 Χ 1 0 " 1.8 X 10~ 0.12

6

— —

6.2 X 10~

_

5

2510

—

—

2 2

2

—

7.5 Χ Ι Ο " 3.7 Χ Ι Ο " 0.13 6.1 Χ Ι Ο " 0.15 0.12 0.32 0.11

— —

2

760

1

— —

4

2

1.0

8.6 Χ Ι Ο " 3.8 Χ 1 0 " 0.11 7.6 Χ Ι Ο " 0.13 0.12 0.33 0.11

6

—

3

6

2840

C H /Air 1.9

2

20

1420 0.73 0.15 1.1 X 10~ 4

2

1.0

40

2250 0.46 0.31 0.23 3.6 Χ 7.3 Χ 7.7 Χ 4.3 X

2

2

2

760

2

C H /0

H /Air 0.40

1.75

b


Typical Flame Temperatures and Equilibrium Compositions"

2 2

2

2239 0.62 2.4 Χ 0.10 2.2 X 7.9 Χ 1.1 X

ΙΟ" 10~ 10" 10 "

2

3 5 6

—

0.25 1.05 Χ Ι Ο " 9.2 X 10~ 2.2 Χ Ι Ο "

2

6

6

™ Concentrations are expressed as mole fractions. Species with concentrations α^η^ο+η-,

trations)

e

3

297

Reactions in Flames

CALCOTE AND JENSEN

e

which is the usual situation,

71H O^CHO 2

4

T h e appropriate concentrations (from Figure 1 at low pressures and from the data of Green and Sugden (19) at 1 atm.) are given i n units of mole cules/ c m . below. 3

2 torr (Figure 1) M a x i m u m H 0 concentration, m i o Measured WCHO E q u i l i b r i u m w ,o +


3

3 Χ 10 1.5 Χ 10 5.8 Χ 1 0

+

3

10

7

+

H

14

760 torr (Green and Sugden) 2 Χ 10 5 Χ 10 1.9 Χ 10

11

5 18

φ Q.

ε
C H ~ + H

-39

(37)

C H~ + Ο

-41

(38)

-25

(39)

2

C - + OH 2

2

C - + H 0 2

-> C H ~ + O H

2

2

Other ions might be produced from C ~ via the following reactions, some of which may rapidly reach equilibrium: Downloaded by UNIV OF ARIZONA on November 21, 2012 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1966-0058.ch017

2

AH, kcal./mole

c c 2

+ OH

— Ο- + C H

L

+ OH

— OH- +

c,-

+ HjO

c,-

+

4.6

2

30

(41)

— OH- + C H

14

(42)

-

37

(43)

C, 2

0

c - + o

o-

— c-

2

(40)

+ C

2

+ CO

-69

(44)

It seems reasonable to suppose that masses 16 and 17, Ο ~ and O H ~, are produced principally by Reactions 40 and 42 while mass 12, C ~, is pro duced by Reaction 44. Some of the other ions in Table I V might be produced via: AH, kcal./mole Or

+ Η

Ο-

Ο-

+ C H 2

+ OH

— OH- + C H

2

2

C H-

+ OH

O H - + C,H

C H2

+ H 0

C HH 0-

OH"

+ CHO

2

Ο"

2

+ CH 0 2

OH-

2

2

(50)

-

(51)

CH 0 3

2

+ CH >0

— CH 0 -

2

2

(48)

2

0 -

CH0 ~ + H 0

(47)

— CH 0 -

2

:

-14

(49)

-* H 0 ~

2

(46)

2.3

2

+ H 0 2

-

— CH0 - + Η

0 2

(45)

2

2

+ CH 0

-71

(52)

3

2

(53)

3

CH0 H 02

(54)

2

Reactions 48-54 are simple attachments of ions to electronegative species, forming ions for which the heats of formation are unknown. T h e ions C H 0 " (mass 45) and C H O H 0 " (mass 63) are evidently not genuine flame ions in Green's system (20); they are produced in the sampling system. Nevertheless, under other conditions they might well be produced in the flame itself. 2

a

2



17.

CALCOTE AND JENSEN

0

2

305

Reactions in Flames

4

6

8

10

12

DISTANCE FROM BURNER, cm Figure 2.

Positive and negative ion profiles in an acetylene/ oxygen flame

Pressure = 1.0 torr; total flow = 70 cc./sec. at STP; equivalence ratio = 1.0; burner diameter = 15 cm.; from Ref. 9.

Thus, there is no great difficulty in accounting for each of the ions in Table I V ; the problem which remains is that of characterizing the rates and equilibrium constants of the various simultaneous reactions. A s expected, the addition to the flame of the electronegative species CI (from HC1) or C N (from C H C N ) produces C I " or C N ~ . A t low pressures, the total negative ion population may be increased (9) while at 1 atm. all the negative ions can be replaced by C l ~ (14). Thus, both direct attachment and charge transfer are involved. 3

Nonhydrocarbon

Flame

Ions

O r i g i n of Ions i n N o n h y d r o c a r b o n F l a m e s . O f flames which do not contain hydrocarbons, only cyanogen/oxygen flames have so far been found to contain levels of " n a t u r a l " ionization comparable with those


306

ION-MOLECULE REACTIONS IN T H E GAS PHASE

observed in hydrocarbon flames (5). T h e only positive ion present in significant quantities in low pressure cyanogen flames is N O (a marked contrast with the situation in hydrocarbon flames). Bulewicz and P a d ley (5) have demonstrated that levels of ionization achieved in cyanogen flames are not consistent with thermal ionization and have suggested that one or more of the following energetically feasible reactions might result in primary ionization. +

2

AH

= - 1 2 kcal./mole

(55)

C N + Ο + Ο — N O + + C O + e~

AH

= - 6 kcal./mole

(56)

NO

C O + N O + + e~

AH

= - 4 2 kcal./mole

(57)

N O + + e~

AH

=

(58)


NO

+ Ν + Ν

+ C + Ο

N O + + e~ + N

T h e reaction Ν + Ο

0

2

4

6

+67 kcal./mole

8


Negative ion profiles in an ethylene/oxygen flame at 2 torr

Equivalence ratio = 1.0; diameter = 15 cm.;

total flow = 70 cc./sec. at STP; burner unpublished work of W. J. Miller.



17.

Reactions in Flames

CALCOTE AND JENSEN

307


Negative ion profiles in an ethylene/oxygen flame at 10 torr

Equivalence ratio — 1.0; total flow = 52 cc./sec. at STP; burner diameter = 7.5 cm.; unpublished work of W. J. Miller.

(analogous to C H + Ο CHO + e ~) apparently accounts for only a small proportion of ionization even in hot ( ^ 4 0 0 0 ° K . ) cyanogen flames. Ionization in other flames (e.g., hydrogen/oxygen/nitrogen flames) appears to stem almost entirely from the presence of hydrocarbon im purities; such reactions as (27) : +

Η + Η + O H -> H 0 + + e~ 3

AH

=

+27

kcal./mole

(59)

are no longer considered to make important contributions (see Discussion of Ref. 5). T h e origin of ions observed in ammonia/oxygen flames also remains a matter for conjecture (12). Ion-Molecule Reactions i n Nonhydrocarbon Flames. Ionmolecule reactions which play important parts in flame ionization phe-


308

I O N - M O L E C U L E REACTIONS I N T H E GAS PHASE

nomena in nonhydrocarbon flames fall into three broad categories: (1) proton-transfer, (2) collisional detachment, and (3) dissociative charge transfer. Examples of such processes are considered below. PROTON

TRANSFER

REACTIONS.

De

Jaegere,

Deckers,

and

van

Tiggelen (12) have observed that the equilibrium


NH

4

+ H 0 ' molecule c m . " at 2 0 0 0 ° K . , and &6i is thus ~10-*e~ > c m . molecule sec. T h e pre-exponential factor i n ku is approximately an order of magnitude higher than the neutral particle collision frequency. Reaction 61 is rather slow (ku < 10 c m . m o l e c u l e s e c . ) , and equilibrium between [ O H ] and [e~] in flames may well be established more rapidly through 6

- 3 0

6

-2

- 1

l

e

41

- 1 3

3

OH

-

mlRT

m RT

-1

-1

-1

-1

-

+ H N O + + H 0 + Η 3

2

Δ Η = +68 kcal./mole

(64)

but later rejected this explanation (see Discussion of Ref. 5) ; the reaction would need to have a very high pre-exponential factor (^40 ~ ) to ex plain the observed rate of N O production. T h e mechanism of forma tion of N O + ions under these conditions remains uncertain, although charge exchange with hydrocarbon ions present as impurities is a pos sibility. 7

+


CALCOTE

17.

Metallic

309

Reactions in Flames

AND JENSEN

Additives

For many metals, adding a small proportion (^4 part in 10 ) of metallic compound to an acetylene/airflame(or to an hydrogen/oxygen/nitrogenflameto whose unburned gases has been added 1 vol. % C H ) at atmospheric pressure results in an above-equilibrium concentration of electrons in the burned gases (6). The equilibrium value of n 7i -, eq., for a metal Me is given by the Saha equation: 6

2

2

e

9

e

= Ζ^9Σ


tog,. / 5 ϊ ^ 1

+3

τ + 15.37 + lo „{g(T)}

l o g l 0

gl

where V is the ionization potential in e.v., and g(T) is a term accounting for electronic partition functions of Me , Me, and e ~. For many metals (e.g., Pb, Cr, Mn), n -/n - ., ~ 10 under the above conditions (6, 24). These observations have been interpreted in terms of a rapid charge transfer reaction +

e

e t

cq

,Me+ + H 0 + Η H 0+ + M e < ^ ^Me + H + OH

(65)

2

or

3

+

(66)

2

The rate constants of these reactions are difficult to measure since the charge transfer is usually completed within a short distance of the re action zone. Soundy and Williams (34) have nevertheless been able to obtain preliminary values for the rate constants, selected values of which appear in Table V. Although the order of magnitude of these Table V . Rate Constants of the Reaction Me + H 0 - ^ M e + Products +

+

3

Metal, Me

AH(kcal./mole)

Pb Mn Cr Li Zn

k (10~ Gb

10

cm. molecule'

+26 +26 +11 -21 +71

3

1

sec.' ) 1

10 8.5 8.0 7.5 1.0

rate constants seems reasonable for reactions of this type, the pattern of results is somewhat puzzling; for example, it is surprising that the exotherLi + H 0 +

Li + + Products

3

(67)

mal reaction appears to be slower than the endothermal process Mn + H 0+ -> Mn+ + Products 3

(68)

However, the fact that lithium hydroxide formation was ignored when & for Li was calculated might account for the low observed value for this metal. Again, both n -/n -, . and kt& appear to achieve maxi mum values for metals with ionization potentials of about 170 kcal./mole whereas the energy available from the reaction 65

e

e

C(l

H 0+ + e~ 3

H 0 +H 2


(15)

310

I O N - M O L E C U L E REACTIONS IN T H E GAS P H A S E

is only about 143 kcal./mole. (The values of AH, the heat change of Reaction 65, listed are based on TaFroze and Frankevich's (38) rather reliable value for the proton affinity of water (168 kcal./mole).) Further more, recent preliminary mass spectrometric studies of H a y hurst and Sugden (22) suggest that the rate of production of sodium ions i n the reaction zone of an H / 0 / N flame to which sodium and 1% acetylene have been added exceeds the rate of disappearance of H 0 +. The possi bilities that ions other than H 0 contribute to charge exchange or even that the observed ionization levels stem from "hot-electron" reactions 2

2

?

3

3

(22,

+

40):


M e + e-*

M e + + 2e~

(69)

therefore cannot be ruled out at present. A t low pressures (^3 torr) metallic ions are produced only slowly i n flames containing hydrocarbons, and at high pressures (^1 atm.) ions are generated too fast to measure isothermal rates accurately. Further work at intermediate pressures might well resolve the questions raised above. Replacing H 0 (or other polynuclear) ions by such species as N a and K has important consequences i n rocket exhaust analyses (32). T h e subsequent recombination reaction (25): 3

+

+

+

N a + + e~ + M

Na + M

(k ~

1.5

X

10

- 2 0

T ) - 2

(70)

provides for a considerably slower rate of electron concentration decay than does Reaction 15, for which k = 2.4 X 10~ c m . m o l e c u l e s e c . Another type of charge exchange reaction proposed recently is (33) Reaction 71. 7

N a + S r O H + -> N a + + S r O H

3

-1

AH ~ 0 kcal./mole

-1

(71)

Sugden and Schofield (33) suggest that this reaction (with a rate constant ~10 c m . m o l e c u l e s e c . ) can account for the boost i n ionization of sodium observed when strontium salts are supplied to flames contain ing sodium. There is evidence (24, 33, 36) which strongly suggests that equilibrium ionization of strontium i n flames is rapidly established v i a - 8

3

-1

-1

SrO + H

Ion-Molecule Reactions in Flames

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