molecule reaction in methane and ammonia mixtures

ARTICLE IN PRESS

Vacuum 78 (2005) 235–240 www.elsevier.com/locate/vacuum

Mass spectrometric study of ion/molecule reaction in methane and ammonia mixtures L. Wo´jcik, A. Markowski Institute of Physics, Maria Curie Sk!odowska University, 20-031 Lublin, Poland

Abstract Ion/molecule reactions initiated by electron impact in ammonia–methane mixtures have been studied by means of quadrupole mass spectrometer with a high-pressure ion source. The total pressure of the mixtures was ranged from 0.5 to 33.3 Pa. Measurements were performed for different concentrations of ammonia and methane in the mixture (from 10% CH4+90% NH3 to 90% CH4+10% NH3 with 10% increment). The major ion/molecule reactions have been identified. Relative ion intensities for the observed ions were determined as a function of total gas pressure inside the ion source collision chamber and concentration of methane in the mixture. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mass spectrometry; Methane; Ammonia; Ion molecule/reaction

1. Introduction Nitrogen, oxygen and argon make up more than 99% of the earth’s atmosphere. Other gases like carbon dioxide, water vapor, oxides of nitrogen, methane and ammonia are considered trace gases. Although their absolute volume is very small, they have a significant effect on the earth’s weather and climate. Methane and ammonia are probably primary constituents of atmospheres in the outer planets and early earth. Ion/molecule reactions involving these gases are very important processes Corresponding author. Fax: +48 81 5376191.

E-mail address: [email protected] (L. Wo´jcik).

especially for the upper layer of the earth atmosphere. Knowledge about mechanisms of these reactions is very important for the protection of the natural environment [1]. The authors present the results of ion/molecule reaction study in the methane–ammonia mixtures. Measurements were made by means of the quadrupole mass spectrometer with the highpressure electron impact ion source constructed by Wo´jcik and Bederski [2]. The gas pressure inside the collision chamber of the ion source was changed in the range from 0.5 to 33.3 Pa. Ion/ molecule reaction has been studied for different concentrations of methane and ammonia in the + mixtures (from 10% CH+ 4 +90% NH3 to 90%

0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.032

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+ CH+ 4 +10% NH3 with 10% methane increment). Primary ions were produced in the collision chamber of the ion source as the result of interaction of electrons with neutral molecules. All measurements were made at the repeller potential 3 V and the electron energy of 300 eV. Ionic processes occurring in pure NH3 [3,4–11], pure methane [2,3,12–22] and in the CH4–NH3 mixtures [23–26] have been studied by many investigators using various mass spectrometric techniques.

2. Experimental Measurements of ion/molecule reactions in the CH4–NH3 methane ammonia mixtures were performed by means of the quadrupole mass spectrometer with the high-pressure ion source. The instrument and techniques have been described previously [10,27–29]. The ammonia and methane used in experiment, supplied by Merck were of the best available purity (99.96% and 99.99%, respectively). Total gas pressure inside the collision chamber of the ion source was changed by means of a precision sapphire valve and controlled by the MKS Baratron capacitance manometer. The quadrupole mass spectrometer is able to perform ion mass analyses in the range from 1 to 400 u. The detection system of this spectrometer consists of the Balzers 16 stage electron multiplier. Ammonia and methane mixtures were made in the independent inlet system and then supplied directly to the ion source collision chamber. The differential vacuum system was applied to the separate ion source and analyzer regions evacuation. The highspeed diffusion pump 2000 l/s was used to evacuate the ion source region and 800 l/s diffusion pump for analyzer evacuation. Mass spectrometer is equipped in special computer interface for data collection and data analyses. In order to correct the errors resulting from ion discrimination effects in different regions of the mass spectrometer, the ion transmission coefficient on the ion path from the source to the collector of an electron multiplier was determined [30]. Energy of ionizing electrons can be changed from 25 to 500 eV.

3. Results and discussion At a relatively low pressure of methane inside the ion source collision chamber, the primary ions + + CH+ 4 , CH3 and CH2 were observed. As the pressure increases, these ions react with neutral molecules of methane to form secondary ions according to the reactions [2,3,12–16,18,19]: þ CHþ 4 þ CH4 ! CH5 þ CH3 ;

(1)

þ CHþ 3 þ CH4 ! C2 H5 þ H2 ;

(2)

þ CHþ 5 þ CH4 ! C2 H5 þ 2H2 ;

(3)

þ CHþ 2 þ CH4 ! C2 H3 þ H2 þ H:

(4)

At low pressure of ammonia inside the ion + source ionization chamber (103 Pa) NH+ 3 , NH2 , + + + 2+ NH , H2 , H , and NH3 ions are produced as the result of ionization and dissociative ionization of ammonia by electrons. Partial ionization crosssections for ammonia bombarded by electron with the energy from 25 to 1000 eV were determined in the earlier works by Bederski et al. [31]. At the mixture pressure inside the ionizing chamber ion + source 1.3 Pa, the NH+ 3 , and NH2 , were observed as the results of primary ionization and dissociative ionization of ammonia by electrons [32]. These primary ions react rapidly with neutral molecules of ammonia and the following ion/ molecule reaction can be observed [3,7–11]. þ NHþ 2 þ NH3 ! NH4 þ NH;

(5)

þ NHþ 3 þ NH3 ! NH4 þ NH2 ;

(6)

NHþ þ NH3 ! NHþ 4 þ N;

(7)

þ NHþ 2 þ NH3 ! NH3 þ NH2 :

(8)

For higher ammonia pressure the NH3NH+ 4 cluster ion can be observed according to the following reaction [32,33]: þ NHþ 4 þ NH3 $ NH4 NH3 :

(9)

The production of cluster ions NH4(NH3)n+ for n41 is also possible; however, they were not observed in the present experiment.

ARTICLE IN PRESS L. Wo´jcik, A. Markowski / Vacuum 78 (2005) 235–240

In the mixtures of CH4–NH3 ion/molecule processes can take place according to the following reactions scheme [23–26]: CHþ 4

NHþ 4

237

þ C2 Hþ 4 þ NH3 ! NH4 þ C2 H3 ;

(15)

þ C2 Hþ 3 þ NH3 ! NH4 þ C2 H2 ;

(16)

þ CH3 ;

(10)

þ CHþ 4 þ NH3 ! CNH3 þ 2H2 ;

(17)

þ NHþ 3 þ CH4 ! NH4 þ CH3 ;

(11)

þ NHþ 3 þ CH4 ! CNH3 þ 2H2 ;

(18)

þ CHþ 3 þ NH3 ! NH4 þ CH2 ;

(12)

þ CHþ 4 þ NH3 ! CNH4 þ H2 þ H;

(19)

þ CHþ 2 þ NH3 ! NH4 þ CH;

(13)

þ CHþ 3 þ NH3 ! CNH4 þ H2 ;

(20)

þ C 2 Hþ 5 þ NH3 ! NH4 þ C2 H4 ;

(14)

þ CHþ 2 þ NH3 ! CNH4 þ H;

(21)

þ NH3 !

20% CH4 + 80% NH3

1.0

20% CH4 + 80% NH3 0.20

0.7 0.6

+

NH4

0.5

+

NH4NH3

0.4 0.3 0.2 0.1

+

+

+

0.14

CH5 (NH3 ) + CNH4

0.12

CNH6

0.16

×3

+

0.10 0.08 0.06 0.04 0.02

0.0

0.00 0

5

10

(a)

15 20 Pressure [Pa]

25

30

35

0

5

10

40% CH4 + 60% NH3

15 20 Pressure [Pa]

25

30

35

40% CH4 + 60% NH3 0.4

1.0 0.9 0.8

Relative intensities Ii/ΣIi

Relative intensities Ii/ΣIi

+

CH4 (NH2 )

0.18

0.8

Relative intensities Ii/ΣIi

Relative intensities Ii/ΣIi

0.9

0.7 +

NH4

0.6

+

NH4NH3

0.5 0.4 0.3 0.2

+

+

+

+

CH4 (NH2 ) 0.3 ×3 0.2

CH5 (NH3 ) + + C2H5 (CNH3 ) CNH4+ +

CNH6

0.1

0.1 0.0

0.0 0 (b)

5

10

15 20 Pressure [Pa]

25

30

35

0

5

10

15 20 Pressure [Pa]

25

30

35

Fig. 1. Relative ion currents as a function of total mixture pressure at constant repeller potential 3 V for two concentrations of methane (a) 20% and (b) 40% in the mixture with ammonia.

ARTICLE IN PRESS L. Wo´jcik, A. Markowski / Vacuum 78 (2005) 235–240

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þ NHþ 3 þ CH4 ! CNH4 þ H2 þ H;

(22)

þ NHþ 2 þ CH4 ! CNH4 þ H2 ;

(23)

þ CHþ 4 þ NH3 ! CNH6 þ H;

(24)

þ NHþ 3 þ CH4 ! CNH6 þ H:

(25)

CH+ Although the reaction between 4 , + + NH3 , NH and CH4 or NH3 appears sufficiently similar, they differ in product and mechanism [24]. Figs. 1 and 2 present the results of ion/molecule reaction measurements for four selected methane concentrations in the mixture with ammonia

1.0

0.40

0.9

0.35

0.8

Relative intensities Ii/ΣIi

Relative intensities Ii/ΣIi

CH+ 3 ,

(20%, 40%, 60% and 80%) at the constant repeller potential 3 V. At a low total pressure of the mixtures, the main primary ions observed formed by elec+ + tron impact are CH+ 4 (and/or NH2 ) and NH3 . + CH4 ion intensities are relatively high especially for high methane concentration of all mixtures (see Fig. 2). As the total mixture pressure increases, decrease + in CH+ 4 and NH3 relative ion currents intensities can be observed. These ions react effectively to form secondary ions and at the pressure 5 Pa they are practically not observed in the mass spectrum.

60% CH4 + 40% NH3

0.7 0.6

NH4+

0.5

+

HN4NH3

0.4 0.3 0.2

+

0.30 0.25 ×3

0.20

+

CH4 (NH2 ) + + CH5 (NH3 ) + C2H5 (CNH3+) CNH4+ + CNH6

0.15 0.10 0.05

0.1 0.0

0.00 0

5

10

(a)

15 20 25 Pressure [Pa]

30

0

35

5

10

0.4

1.0 0.9

80% CH4 + 20% NH3

0.8 0.7

Relative intensities Ii/ΣIi

Relative intensities Ii/ΣIi

60% CH4 + 40% NH3

+

0.6

NH4 + NH4HN3

0.5

+

CH4

0.4 0.3 0.2

15 20 25 Pressure [Pa]

30

35

80% CH4 + 20% NH3 +

+

CH5 (NH3 )

0.3

+

+

C2H5 (CNH3 ) +

×3

0.2

CNH4 + CNH6

0.1

0.1 0.0

0.0 0

(b)

5

10

15 20 25 Pressure [Pa]

30

35

0

5

10

15 20 25 Pressure [Pa]

30

35

Fig. 2. Relative ion currents as a function of total mixture pressure at constant repeller potential 3 V for the two concentrations of methane (a) 60% and (b) 80% in the mixture with ammonia.

ARTICLE IN PRESS L. Wo´jcik, A. Markowski / Vacuum 78 (2005) 235–240

For the mixtures with a high methane concentration and the total gas pressure 3 Pa the secondary ions at m=q ¼ 17 come from reaction (1). These ions can interfere with NH+ 3 ions at a certain mixture pressure. The secondary ions + + + NH+ and/or C2H+ 4 , CNH3 5 , CNH4 , CNH6 + and NH4NH3 at m=q ¼ 18; 29, 30, 32 and 35 were observed as the result of ion/molecule reactions. Relative ion currents formed by NH+ 4 ions at m=q ¼ 18 as a function of total mixture pressure have a maximum for all investigated mixture concentrations. The reason for such a maximum is an increase of ion/molecule reaction probabilities with the total pressure increasing and formation of cluster ions NH4NH+ 3 (reaction (9)). A shift of this maximum in the direction of higher total mixture pressures can be observed as the concentration of ammonia in the mixture decreases. The relative ion currents at m=q ¼ 29 are + formed both from the CNH+ 3 and C2H5 ions. These ions come from reactions (2), (3) and (17), (18). The first two reactions predominate, when the concentration of CH4 in the mixture is high. Behavior of relative ion currents for m=q ¼ 29 and at 17 as a function of gas pressure in a lowpressure range is quite similar. It suggests that both these ions are produced as the result of reactions (1) and (3) in methane. As the total gas pressure is getting higher, C2H+ 5 ions ðm=q ¼ 29Þ can react with ammonia (reaction (14)). + Only a small amount of CNH+ 4 and CNH6 was observed in experiment as the result of ion/ molecule reaction (19)–(23) and (24), (25), respectively. These reactions engaged both ammonia and methane primary ions. Relative currents formed + by CNH+ 4 and CNH6 ions have a maximum shifted in the direction of high total gas pressures, as the concentration of ammonia in mixtures + + decreases. The primary ions NH+ 3 , NH2 , CH4 , + + CH3 , CH2 are consumed in reactions (10)–(13) leading to NH+ 4 ions production. Figs. 3a and b present the three-dimensional + plots of NH+ 4 and NH4NH3 relative ion currents as a function of total mixture pressure and concentration of ammonia.

239

+ Fig. 3. Fractional abundance of NH+ 4 and NH4NH3 ions as a function of pressure and concentration of ammonia in the mixtures.

The NH+ 4 ion currents have a maximum shifted in the high-pressure direction when the concentration of ammonia in the mixtures decreases. For the mixture pressure higher than 18 Pa, the NH+ 4 ion currents do not depend on the mixture concentration. NH4NH+ 3 cluster ions reached their maximum both for the high total pressure of the mixture and concentration of ammonia in the mixture. 4. Conclusions The purpose of this paper was to examine ion/ molecule reaction mechanisms in the mixtures of methane and ammonia. As the result of ammonia and methane ionization by energetic electrons, the

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+ + + + primary ions CH+ 4 , CH3 , CH2 and NH3 , NH2 , + NH were produced. These ions react with neutral methane and ammonia to form secondary ions. Measurements were performed for different concentrations of ammonia and methane in the mixtures in a wide range of total mixture pressures. The main secondary ions observed were + + + NH+ 4 , cluster ions NH4NH3 and CH5 , C2H5 + + (and /or CNH3 ). Small amounts of CNH4 and CNH+ were also registered. The scheme of 6 reactions that provide secondary ion production is presented. Relative ion currents for the main secondary ions observed NH+ 4 has a maximum that comes from the reactions of cluster ions + NH4NH+ 3 formation. The maximum for NH4 secondary ions is shifted in the direction of higher total mixtures pressures as the concentration of ammonia in mixtures decreases.

References [1] Manahan SE. Environmental chemistry. Michigan: Lewis Publishers; 1991. [2] Wo´jcik L, Bederski K. Ann Univ Mariae Curie–Sk"odowska Sect AAA 1988, 1989;43, 44:365. [3] Gupta SK, Jones EG, Harrison AG, Myher JJ. Can J Chem 1967;45:3107. [4] Ryan KR, Futrell JH. J Chem Phys 1965;42:824. [5] Ryan KR, Futrell JH. J Chem Phys 1965;43:3009. [6] Harrison AG, Thynne JCJ. Trans Faraday Soc 1966;62:2804. [7] Chupka WA, Russell E. J Chem Phys 1968;48:1527. [8] Marx R, Mauclaire G. Int J Mass Spectrom Ion Phys 1973;10:213. [9] Huntres WT, Pinizzotto RF. J Chem Phys 1973;59:4742.

[10] Wo´jcik L, Bederski K. Int J Mass Spectrom Ion Processes 1993;127:11. [11] Ryan KR. J Chem Phys 1970;53:3844. [12] Field FH, Franklin JL, Lampe FW. J Am Chem Soc 1957;79:2419. [13] Wexler S, Jesse N. J Amer Soc 1962;84:3425. [14] Field FH, Franklin JL, Munson MSB. J Am Chem Soc 1963;85:3575. [15] Field FH, Munson MSB. J Am Chem Soc 1965;87:3289. [16] Abramson FP, Futrell JH. J Chem Phys 1966;45:1925. [17] Studniarz SA, Franklin JL. J Chem Phys 1968;49:319. [18] Clow RP, Futrell JH. Int J Mass Spectrom Ion Phys 1970;4:165. [19] Herod AA, Harrison AG. Int J Mass Spectrom Ion Phys 1970;4:415. [20] McIver RT, Dunbar RC. Int J Mass Spectrom Ion Phys 1971;7:471. [21] Anicich VG, Bowers MT. Int J Mass Spectrom Ion Phys 1973;11:329. [22] Michalak L, Adamczyk B. Int J Mass Spectrom Ion Process 1988;85:319. [23] Huntress Jr WT, Pinizzotto Jr RF, Laudenslager JB. J Am Chem Soc 1973;95:4107. [24] Smith GPK, Saunders M, Cross Jr RJ. J Am Chem Soc 1976;98:1324. [25] Huntress Jr WT, Elleman DD. J Am Chem Soc 1970;92:3565. [26] Munson MSB, Field FH. J Am Chem Soc 1965;87:4242. [27] Wo´jcik L. Ann Univ Mariae Curie–Sk"odowska Sect AAA 1999;54:74. [28] Wo´jcik L, Markowski A. Vacuum 2003;70:391. [29] Wo´jcik L, Markowski A. Pr Nauk Politech Warsz Elektron 2002;143:149. [30] Wo´jcik L, Bederski K. Int J Mass Spectrom Ion Process 1996;153:139. [31] Bederski K, Wo´jcik L, Adamczyk B. Int J Mass Spectrom Ion Phys 1980;35:171. [32] Wincel H. Int J Mass Spectrom Ion Phys 1972;9:267. [33] Hancock RA, Hodges MG. Int J Mass Spectrom Ion Phys 1983;46:329.