Ionization of the group 3 metals La, Y and Sc in H2O2Ar flames

ELSEVIER

International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76"

and Ion Processes

Ionization of the group 3 metals La, Y and Sc in H2-O2-Ar flames Patricia M. P a t t e r s o n , J o h n M. G o o d i n g s * Department of Chemistry, York University, 4700 Keels Street, North York, Ont. MSJ 1P3, Canada

Received 1 February 1995; accepted 26 April 1995

Abstract

Four pairs of premixed, fuel-rich/fuel-lean (FR/FL; equivalence ratio ~ = 1.5/0.75), H 2 O 2 - A r flames at four temperatures in the range 1900-2425 K, all at atmospheric pressure, were doped with about 10-6 mole fraction of the group 3 metals La, Y and Sc using atomizer techniques. The metals produce solid particles in the flames and gaseous metallic species. The latter include free metallic atoms, A, near the flame reaction zone, but only the monoxide AO and the oxidehydroxide OAOH further downstream at equilibrium; the [OAOH]/[AO] ratio varies in F R / F L flames. Metallic ions (< 1% of the total metal) were observed by sampling a given flame along its axis through a nozzle into a mass spectrometer. All of the observed ions can be represented by an oxide ion series AO +.nH20 (n = 0 3 or more) although their actual structures may be different; e.g. A(OH) + for n = 1, interpreted as protonated OAOH. A major objective was to ascertain the ionization mechanism, principally that of La. The ionization appears to receive an initial boost from the exothermic chemi-ionization reaction of A with atomic O to produce AO+; further downstream, the ionization level is sustained by the thermal (collisional) ionization of AO to produce AO + and/or the chemi-ionization of OAOH with H to produce A(OH)~-. The ions AO +, A(OH)~- and higher hydrates are all rapidly equilibrated by three-body association reactions withwater. Ions are lost by dissociative electron ion recombination of A(OH)~ and possibly higher hydrates. The chemical ionization of the metallic species by H3 O+ was investigated by adding a small quantity of CH4 to the flames. The ion chemistry is discussed in detail. An estimate of the bond dissociation energy D0(OLa-OH ) = 408 + 40 kJ tool -1 (4.23 ± 0.41 eV) was obtained. Keywords: Chemi-ionization; Flame ionization; Group 3 metals; Lanthanum; Mass spectrometry

1. Introduction

Over the past forty years, many studies of the ionization of metals in flames have been carried out. Our interest in the flame ionization of lanthanum stems from the work of Dyke and his colleagues [1 5], who observed the chemi-ionization of lanthanide metal atoms Ln (La, Ce, Pr, Nd, Sm, Eu, Gd) * Corresponding author.

reacting with the oxygen species o(Sp) and 0 2 ( X 3 Y]g, a l a g ) . T h e s e p r o c e s s e s w e r e i n v e s t i g a t e d near room temperature by chemi-elec-

tron spectroscopy, and chemi-ion spectroscopy with mass analysis for identification of the ions. The metal was evaporated from an r.f.heated furnace and crossed with the oxidant in a reaction cell. Clemmer et al. [6] have discussed the ion and neutral thermochemistry of the gaseous oxides of La, and also those of Y and Sc. Specifically, they measured the bond

0168-1176/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0168-1176(95)04220-2

56

P.M. Patterson, J.M. Goodings/lnternational Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

energies and ionization energies of these group 3 metal dioxides using a guided ion beam mass spectrometer. Clearly, a flame as a chemical reactor is a very different medium, both as to temperature and the reacting species which are present. The interest in the flame ionization of the group 3 metals arises because the two s electrons in the metal atom A form a very strong bond with oxygen in the gas phase, leaving an odd and easily ionizable d electron. Thus, the chemiionization reaction of A with O to form AO + is exothermic for all three metals. Many years ago, Kelly and Padley [7] studied the total ionization without mass analysis of 14 different metals in H z - O z - N 2 flames, La and Y among them, using a rotating single probe, and found relatively high levels of ionization for La. A major consideration for some of these metals was the presence of involatile oxide particles. Recently, we reported preliminary results for H 2 - O z - A r flames doped with La, used as a test system to examine the operating characteristics of a new flame-ion mass spectrometer [8]. Earlier, we had studied the chemical ionization (CI) of the first row of 10 transition metals, including Sc, in a CH 402 flame using our original flame-ion mass spectrometer [9]; the major Sc ions observed were ScO + . n H 2 0 (n = 0-3). Our main objective in this study is to ascertain how lanthanum becomes ionized in H 2 O2-Ar flames where the level of natural ionization is low; that is, chemical ionization of lanthanum species by H3 O+, the dominant ion downstream in a hydrocarbon flame, is not operative. Because of their chemical similarity to La, observations have also been made with the other group 3 metals Y and Sc. They provide a useful gradation of properties for diagnosis of the ionization mechanisms. Accordingly, a variety of fuel-rich (FR) and fuel-lean (FL, i.e. oxygen-rich) flames covering a range of temperatures has been doped with La, Y and Sc for analysis of the metallic ions

by sampling the flame gas through a nozzle into a mass spectrometer. In all cases, a series of ions corresponding to AO +-nH20 (n = 0-3 or 4) was observed, differing mainly in their relative magnitudes and the shapes of their concentration profiles in the flames. In addition, it has been advantageous to add a small quantity of a hydrocarbon (CH4) which produces a considerable concentration of H30 + in these flames. The chemical ionization (CI) of metallic neutral species by H30 + via proton transfer and/or charge transfer provides useful information for diagnostic purposes. A problem immediately arises in explaining the ionization mechanism because the known chemi-ionization reactions for these metals involve atomic A, but A is a very minor species, at least downstream in these flames. However, the ionization level observed downstream is relatively high, such that a different ionization mechanism must be invoked. The collisional (thermal) ionization of AO is the obvious choice, but it appears to be supplemented by a chemi-ionization reaction of the oxide-hydroxide OAOH with H atoms. Thus, it is our aim in this communication to substantiate a model whereby the ionization receives an initial boost near the flame reaction zone from chemi-ionization of A reacting with O atoms, and the ionization level is then sustained downstream by the thermal ionization of AO and the chemi-ionization of OAOH with H. An understanding of La ionization was necessary before we could proceed with our current study, which involves further lanthanide metals (Ce, Pr, Nd) in contrast with La.

2. Experimental Eight premixed, laminar, H 2 - O 2 - A r flames at atmospheric pressure were used in this work: four flames A, B, C and D of fuel-rich composition (FR; equivalence ratio 4~ = 1.5),

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

o--o

~

d d d d d d d

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a-.n

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c~ o

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c~ c~ c5 c5 o

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r..) F, tt~ ~

--,,60

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t"q t'q

~t/'~ ee'~

~

07

eq

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8

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58

P.M. Patterson, J.M. Goodings/lnternational Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

and four flames A 1, B~, C ~ and D' of fuel-lean composition (FL; ~ = 0.75). The eight flames comprise four pairs, each pair consisting of one FR and one FL at approximately equal temperatures; A/A' (at 1900 K), B/B' (2150 K), C/C' (2300 K) and D/D' (2425 K). Their properties are given in Table 1. The compositions were calculated with a computer program which assumes chemical equilibrium in the burnt gas at the adiabatic flame temperature using thermodynamic values based on the J A N A F Tables [10]. These pseudo-onedimensional (flat) flames are stabilized on a water-cooled brass burner described previously [~,], consisting of 151 stainless steel hypode~tiiaic needle tubes in a close-packed circular array of diameter 10.5 mm. It is significant that, when viewed from the side, the burner tubes line up in straight rows. The FR flames were roughly cylindrical in shape, presumably because of afterburning of the excess fuel with atmospheric oxygen downstream around the flame's perimeter. In contrast, the FL flames were slightly conical, tapering inwards downstream, presumably caused by the entrainment of atmospheric air. All flames were doped with the metals by introducing an aerosol of a metallic salt into the premixed gas feeding the burner. The aerosol was formed by spraying an aqueous solution of the salt using an atomizer described previously [8,11]. The solution concentration was usually 0.1 M, and the atomizer provided a total metal concentration in the unburnt flame gas of ~ 10 -6 mole fraction. The salts employed were ScC13 (Aldrich, 99.99% pure) and YC13.6H20 (Aldrich, 99.9% pure); LaC13.7H20 (Aldrich, 99.999% pure) was used initially, but was later replaced by La(NO3)3.6H20 (Aldrich, 99.999% pure). When the lanthanum chloride was used, the La ion signals were extremely noisy on occasion, and exhibited "spikes" on the profile data which were sometimes as large as the normal signal magnitude. The frequency of

the spikes increased with increasing time spent using the mass spectrometer. They were not present when the nitrate was employed. Lanthanum forms solid particles in flames, presumably La203(s), and these will occur with both the chloride and the nitrate. However, it does not follow that the solid oxide particles formed from the two salts will have the same size. On sampling, the solid particles speed up towards sonic velocity in the nozzle throat, with kinetic energies estimated to be in the keV range. If such an energetic particle strikes a metal electrode in the mass spectrometer, a burst of ions could result. If these ions were collected, a signal "spike" would be observed. Perhaps the particle size is optimum with the chloride to enhance this phenomenon. No better explanation can be offered at the present time. The phenomenon has never been observed with other salt solutions in the atomizer. All eight flames doped with these metals were beautifully coloured. The depth of colour was more pronounced the hotter the flame and, for each pair of flames at the same temperature, the FL flame was a paler version of the corresponding FR one. With La as additive, the flames were pale green and the distinct, individual, conical reaction zones at the mouth of each of the 151 burner tubes had a bluish cast. Each cone produced a whitish "stripe", presumably arising from incandescent solid particles, which was visible along the complete length of the flame. These stripes were clearly visible when the observer's eye was oriented normal to the flame axis to look along a row of burner tubes in the circular array. After the flame had been burning for a few minutes, white oxide condensed onto the cold sampling plate in a distorted hexagonal pattern, each hexagon being associated with one burner tube. The flames doped with Y were reddish/pink; the conical reaction zones had a lighter colour, and extended an additional 2-3 mm into the burnt gas. These

P.M. Patterson, J.M. Goodings/lnternational Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

extended reaction zone cones would seem to be associated with solid particle formation. White oxide condensed onto the sampling plate in a pattern similar to that for La but the a m o u n t was less. With Sc, the flames were coloured fuchsia. They exhibited the same extended reaction zone cones observed with Y. A similar pattern of white oxide condensed on the sampling plate, but the quantity was still less than that with Y. The whitish stripes were not clearly visible with Y and Sc, perhaps because the flames were more deeply coloured than those doped with La. However, the similar hexagonal condensation pattern would indicate that solid particles were, nevertheless, streaming from each individual burner tube. The eight H 2 - O 2 - A r flames described in Table 1 exhibit only a very low level of natural ionization. For diagnostic purposes, it was sometimes advantageous to add ~ 0.2 mol% of CH 4 (0.5 cm 3 s -1 in 250 cm 3 s -l total flow rate), which produces a high initial concentration of H 3 0 + ions by chemi-ionization. In this way, chemical ionization of metallic species by H3 O+ could be investigated. The addition is small enough, however, so that the bulk flame composition and temperature remain essentially unchanged. The burner is m o u n t e d horizontally on a motorized carriage with calibrated drive, and the flame axis is accurately aligned with the sampling nozzle of the mass spectrometer. The apparatus has been described in detail previously [8], so only a brief description will be given here. Flame gas containing ions is sampled through an orifice of diameter 0.17 ram. The circular orifice is contained in a tiny electron microscope lens of Pt/Ir alloy swaged into the tip of a conical nozzle mounted in a water-cooled plate made of stainless steel. The ions enter a first vacuum chamber maintained at 0.04 Pa (3 x 10 - 4 Torr), and are formed into a beam by an electrostatic lens. The beam then passes through a 3 m m orifice in the tip of a nose-cone into a

59

second vacuum chamber pumped to a pressure below 0.003 Pa (2 x 10 -5 Torr). The ions traverse a second ion lens into a quadrupole mass filter in which they have an axial ion energy of 15 eV. They are detected by a F a r a d a y collector connected to a vibrating-reed electrometer having a grid-leak resistance of 10 l° f2. Thus, ion signal magnitudes quoted in the figures below as a voltage (in mV) refer to the collected ion current passing through 10 ~° f2. By advancing the flame onto the sampling nozzle, profiles m a y be obtained of an individual ion signal versus distance along the flame axis z. The zero on the distance scale (z = 0) is defined experimentally where the pressure abruptly rises when the sampling nozzle pokes through the flame reaction zones into the cooler unburnt gas upstream. The pressure is measured with an ionization gauge mounted in the second vacuum chamber. As an alternative to individual ions, total ion profiles can be measured (total positive ions, TPI) if the d.c. voltages to the quadrupole rods are switched off. Still with the d.c. voltages switched off and the mass dial set to a given mass number, all of the ions above that mass number are collected; e.g. TPI100 is used to designate total positive ions above a mass number of 100 u. This technique is useful in separating total group 3 metal ions from ions of low mass number such as H3 O+ and minor impurities like Na + and K + if present. Finally, it should be pointed out that the sensitivity of the mass spectrometer is different for individual ions and TPI, as the former are measured at fairly high resolution whereas total ion collection amounts to zero resolution. The former sensitivity is approximately two-thirds of the latter. When flame gas is sampled by the nozzle it cools, first in passage through the cold boundary layer surrounding the orifice [12], and secondly in the expansion to supersonic velocities which occurs downstream of the nozzle throat [13]. As a result, a fast ion/molecule reaction

60

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

near equilibrium can shift in the exothermic direction during sampling. One example is hydration reactions of ions by water [14]. In the case of H3 O+, the hydrates are not believed to be genuine flame ions but arise mainly due to cooling during the sampling process [15]. In the present study of group 3 metals, some of the AO+.nH2 O ions observed are unlikely to be simple hydrates, and their signal magnitudes were much greater than that of the nude AO + ion in all three cases. Therefore the major AO +.nH20 ion signals are thought to represent true flame ions, although their relative magnitudes may be enhanced by sampiing cooling. This conclusion is strengthened by observations of the same flames doped with barium using the same sampling nozzle [16]. A very large BaOH + signal was measured but only its first hydrate BaOH+.H2 O was detected, and then only to the extent of about 1% of the parent ion. A final point should be mentioned in connection with H2 O2-Ar flames. The free radicals H, OH and O are formed in the reaction zone by fast bimolecular processes which include two chain-branching reactions (H+O2~OH+O and O + H 2 ~ O H + H ) and a propagating reaction ( O H + H 2 ~ H20 ÷ H) [17,18]. These reactions are assumed to be balanced in the sense that a balanced reaction proceeds rapidly in opposite directions at negligibly different rates. The radicals are lost downstream in the burnt gas by relatively slow three-body recombination reactions (primarily H + H + M ~ H 2 ÷ M and H + OH + M ~ H 2 0 + M, where M is a third body) [17-19]. As a result, the radicals achieve superequilibrium concentrations in the reaction zone and then fall more or less slowly to their equilibrium values in the burnt gas [17-20]; the phenomenon is sometimes called "radical overshoot". This situation gives rise to the concept of partial equilibrium [21], i.e. the radical concentrations maintain their equilibrium ratios via the fast bimolecular

reactions while they decay towards full thermodynamic equilibrium at the flame temperature [22]. In the cooler flames A/A', the overshoot is large and the radical concentrations do not quite achieve their final equilibrium levels even at z = 30 millimetres downstream. In the hotter flames D/D', however, the degree of overshoot is less, and thermodynamic equilibrium is achieved several m m downstream of z -- 0. If a disequilibrium parameter 7 [23] is defined to represent [OH](actual)/[OH](equilibrium), 7 0 H - 7 in both F R and FL flames; 7 can have values of 100 or more. Also, 7H = 7 in F R flames, but 7H = 73 in FL ones. However, 7o = 72 for oxygen in both F R and FL flames. Where we are concerned below with chemi-ionization reactions of a metal A with atomic oxygen O, the point to be made is that the O radical has the s a m e 7 2 dependence for radical overshoot in F R and FL flames. For the possible reaction of OAOH with H, however, the 3' dependence is very different.

3. Metallic neutral species in flames Atomized metals can react with neutral constituents of the flame gas to form gaseous molecular compounds such as oxides and hydroxides. The group 3 metals La, Y and Sc exhibit a strong tendency towards oxide formation. Their monoxides AO have very high bond dissociation energies between 8.3 and 7.0 eV owing to back-donation of electron density from the O atom, but low ionization energies, because the odd d electron is only weakly bound. These and other thermodynamic data for the group 3 metals are assembled in Table 2. As a consequence of the oxide formation, the equilibrium concentrations of the free atom A for these elements are expected to be almost negligible at the flame temperatures employed in this study. The +3 oxidation state of La, Y and Sc might also favour other

P.M. Patterson, J.M. Goodings/International Journal o f Mass Spectrometry and Ion Processes 148 (1995) 55-76

61

Table 2 Metal and metal oxide thermochemistry at 2400 K Quantity/species

La

IE°(eV) D~(AO) (eV) D~(OA O H ) (eV) -(G~,r /~29s)/T(Jmol J K -l) H~0 - H~298 (kJ mol - j ) G* (kJ m o l - l ) g AG~, r ( k J m o l 1) A/~f.0 ( k J m o l I) AH~f.0(OAOH) (kJ mol l)i

5.58 a 8.27 c 4.20 d -215.685 e -6.314 f -511.33 147.684 e 431.314 a -486.488

a Ref. [24]. bRef. g G* - T[(G°T - I ~ o ) / r Other necessary values from A/~r.0(OAOH ) =

LaO

Y 4.90 a

-282.748 c -8.876 ~ -699.719 _272.594 h _119.64 b

6.22 a 7.41 c 4.14 d -207.96 e -6.858 f -492.246 124.724 e 420.700 a -408.339

YO 5.85 a

-276.079 c -8.844 ~ -653.746 _215.692 h _47.28 t, -

Sc

ScO

6.56 b 7.01 c 3.95 d -200.48 e -7.004 r -474.148 72.572 e 376.315 a -395.797

6.43 b -266.283 c -8.791 ~ -630.288 _223.886 h _53.07 b

[6]. CRef. [25]. dEstimated using D~(OA O) values from Ref. [6]. eRef. [26] rRef. [27]. J = T[-(G~, T - H~298)/T ] - (H~0 -/~298). hAG~.T(A) 4~ 1/203(O2) -- O~)(ao) + G*(AO) - G*(A) - 1/2G*(O2). are G*(O2) calculated using data from Ref. [10], and D ~ ( O 2 ) = 493.522kJmol -l from Ref. [28]. i Estimated AH~f,0(OH ) + A/4~r,0(OH ) - D~(OA OH), using A/4~f,0(OH ) = 38.39 kJ mo1-1 from Ref. [10].

compounds such as the oxide-hydroxide OAOH and the trihydroxide A(OH)3, and possibly other hydroxides AOH and A(OH)2 as well, depending on the reducing or oxidizing nature of a particular flame. In addition, the group 3 metals form solid sesquioxides A203(s ) which have high melting points and boiling points (e.g. La203: melting point, 2307°C; boiling point, 4200°C [29]). Thus an unknown but appreciable fraction of the total metal can be expected to exist as solid particles in these flames at temperatures in the range 1900-2425 K. When vaporized, the solids yield the monoxide AO [30-32]. Estimates can be made for the equilibrium ratios of metallic neutral species concentrations at the various flame temperatures. For example, the ratio [La]/[LaO] can be calculated from the equilibrium constant for the reaction La + H20 ~ LaO + H 2

(1)

using AG~ values given in Table 2 and that for H20 from the JANAF Tables [10]; the equilibrium concentrations for [H20] and [H2] for a particular flame are given in Table 1. These calculations indicate [La]/[LaO] ratios in the range 10-7- I0-12 such that the concentration [La] of the free metal atom is negligible under

equilibrium conditions. The same conclusion is reached for Y and Sc. For the oxide-hydroxide, estimates of the [OLaOH]/[LaO] ratio were made using the equilibrium reaction LaO + H20 ~-~OLaOH + H

(2)

and the values for [H2 O] and [H] given in Table 1. The value of A//~f,0(OLaOH ) was estimated by assuming D~(OLa OH) equal to D~(OLaO) given by Clemmer et al. [6]. Together with A/-/~f.0 values for LaO [6], H20 and H [10], a very approximate concentration ratio was found from l n K = - A H ~ ) ( 2 ) / R T . A similar procedure was followed for Y and Sc. The conclusion reached was that OAOH is a significant neutral species in these flames, having a concentration of approximately the same order of magnitude as that of AO. The [OAOH]/[AO] ratio increases with decreasing flame temperature and, for a given pair of flames at the same temperature, the ratio is higher in the FL flame because [HI is lower. For example, the estimated ratio for La, Y and Sc is, respectively, 0.46, 0.34 and 0.14 in the FR flame D and 2.2, 1.7 and 0.66 in the FL flame D'. Similarly, rough order-of-magnitude calculations for the concentrations of the hydroxide species LaOH, La(OH)2 and La(OH)3 were

62

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

based on the reactions La + H20 ~ LaOH + H

(3)

LaOH + H20 ~- La(OH)2 + H

(4)

La(OH)2 + H20 ~ La(OH)3 + H

(5)

with crude estimates of the relevant bond strengths. The calculations indicate that all of the hydroxides have concentrations comparable with that of atomic La, i.e. they are negligible species in these flames. For example, [La(OH)3]/[LaO] in flame D was estimated to be 10-8. The same conclusion also holds for Y and Sc. For the present study, the important point to be emphasized is that all of the estimates given so far about the concentrations of neutral species refer to equilibrium conditions in the burnt gas at the adiabatic flame temperature. These estimates are definitely not applicable to the flame reaction zone, nor to the early part of the burnt-gas region downstream. The metal introduced into the flame comes from an aqueous solution of a metallic salt (nitrate or chloride) using the atomizer technique. The atomizer forms tiny droplets of aqueous solution which are introduced into the main premixed gas flow of H 2 0 2Ar, which is dry. Thus the droplets evaporate, leaving behind tiny crystallites of the salt hydrate, which pass through the burner into the preheat region of the flame. There they start to decompose, and further dissociate to form atoms in the reaction zone when the temperature is sufficiently high. Subsequently, the atoms react with flame species to form metallic compounds further downstream in the burnt gas, eventually reaching their equilibrium concentration levels. The compounds are usually gaseous but may, as with the group 3 metals, form solid particles also, or even liquid droplets. The point is that there should be a region in or near the flame reaction zone where the concentration of group 3 metal atoms [A]

might be quite high, even though [A] becomes negligible further downstream at equilibrium.

4. Results and discussion

4.1. Preliminary observations Fig. 1 gives mass spectra for the F R flame D with the atomizer spraying a 0.1 M solution of (a) La(NO3)3.6H20, (b) YC13-6H20, and (c) ScC13. In each case, the flame was sampled at z = 15 m m downstream. Each spectrum shows peaks at four mass numbers separated by 18 u corresponding to an oxide ion series A O + . n H 2 0 (n = 0 3 or 4). These ions are unlikely to be simple hydrates. Alternative 400 300

(al La ions

200 100 0 M15S +

A

E

M173 +

M191 +

M209 +

M227 +

M141+

M15g+

M177+

M79+

M97+

Ml15 ÷

M133"

AO+.H;O

AO+.2H20

AO+.3H2Q

AO+.4HzO

200

(b) 150

Y ions

100

C: .m (/} tO

loo] 50

0

M1'05 ÷

J

M123+

l

(c)

75

Sc ions

50 25 0 M61 + AO +

I

I

Mass number (u) Fig. 1. Mass spectra observed at a location z = 15 mm downstream in the fuel-rich flame D with the atomizer spraying 0.1 M aqueous salt solutions of (a) La, (b) Y and (c) Sc.

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

63

Table 3 Chemical/structural interpretation of the AO + • n H 2 0 ion series Mass number

Oxide ions

M M M M M

AO + AO + . AO +AO +. AO +.

+ + + +

18 36 54 72

H20 2H20 3H20 4H20

Dihydroxide ions

Protonated oxidehydroxide ions

Protonated trihydroxide ions

A(OH) + A(OH)2(H20) + A(OH)z(H20)2 ~ A(OH)z(H20) ~

HOA(O)H + HOA(O)H +- H 2 0 HOA(O)H + • 2 H 2 0 HOA(O)H + • 3 H 2 0

A(OH)3 H+ A(OH)3 H +- H 2 0 A(OH)3 H+ • 2 H 2 0

chemical interpretations and formulae are given in Table 3, some of which represent more likely structures for the ions. Basically, these include hydrates of the dihydroxide ion, also interpreted as the protonated oxidehydroxide, and the protonated trihydroxide. Presumably an ion such as La(OH)2(H20) + has a tetrahedral structure with two OH groups and two coordinated water molecules around the central metal atom. The AO+.4H2 O ion detected for Y and Sc has a much smaller signal magnitude than the lower hydrates, and might be interpreted as an actual hydrate with the water molecule lying outside the coordination sphere of the central atom, in keeping with the octet rule. Also, a progression is evident in the signal magnitudes of the A O + . n H 2 0 ions for the three metals in that the largest signal was obtained with n = 1, 2 and 3 for La, Y and Sc, respectively. For the other F R flames (C, B and A in Table 1) at lower temperatures sampled under identical conditions, the distribution of the signal magnitudes for each A O + . n H 2 0 series shifts progressively towards higher n as the temperature is lowered. The FL flame D ~ exhibits a distribution with n very similar to that shown for flame D in Fig. 1 at the same temperature, except that corresponding signal sizes are roughly half as large. The same trends are evident in the other FL flames C t, B ~ and A'; the n distribution is approximately the same for each pair of flames at the same temperature (A'/A, B'/B, etc.), and the corresponding signal magnitudes are always

a factor of 2-4 smaller in the FL flame compared with the F R one. 4.2. Ionization reactions and ion chemistry

Of obvious importance for consideration are the chemi-ionization reactions of the metal atom with oxygen studied by Dyke's group at the University of Southampton [1-5] A + O ~,_~__AO+ + e AHg = IE°(AO) - D°(AO) A + 02 ~ AO~- + e-

(6) (7)

It is worth stating at the outset that no mass spectrometric evidence was ever obtained in this study for the presence of AO~- ions for any of the metals. Reaction (7) is endothermic at flame temperatures according to the discussion of Clemmer et al. [6]. For lack of any evidence, the reaction will not be considered further. The group 3 metals form very strong A - O bonds and the odd electron is easily ionizable; reaction (6) is exothermic if the bond energy exceeds the ionization energy. Once AO + ions are produced, the higher members of the AO +.nH20 series are readily formed by fast three-body hydration reactions with water, the major reaction product in all of the flames A O + . ( n - 1)H20 + H20 + M AO + . n H 2 0 + M

(8)

where M is a third body. The facts that the odd

64

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

electron in AO is only weakly bound and that the monoxide is a major metallic neutral species, at least at equilibrium, mean that thermal (collisional) ionization of AO AO + M ~,-~-AO + + e - + M AH~ = IE°(AO)

(9)

might be expected to proceed with a high reaction rate. The possibility raised in the previous section that the oxide-hydroxide OAOH might also be a major metallic neutral species leads to the consideration of another chemi-ionization reaction OAOH + H ~ - e - + A(OH)y or A O + . H 2 0 AH~0 = IE ° (H) - PA ° (OAOH)

(lO) where PA°(OAOH) is the proton affinity of OAOH. As IE°(H) has such a high value (13.598 eV = 1312.0 kJ tool -1 [33]), an even larger value for PA°(OAOH) would be necessary if reaction (10) were to be exothermic. This would place PA°(OAOH) right near the top of the known proton affinity scale [34]. However, there is a counterpart to reaction (10) for the alkaline earth metals Ba, Sr and Ca, which are one atomic number lower than La, Y and Sc, respectively, in the periodic table. The alkaline earths undergo chemiionization in hydrogen flames to form AOH + ions, either by reaction of AO + H or A + OH [35,36]; the two channels cannot be distinguished. Using values from the J A N A F Tables at 2400 K [10], the AO + H reaction for Ba, Sr and Ca is endothermic by 216, 146 and 136 kJ mo1-1 , respectively. Furthermore, high PA values have been determined for BaO, SrO and CaO by Murad [37] (e.g. 1216 kJ mol -l for BaO). In summary, reaction (10) may be feasible if the chemistry of OAOH for the group 3 metals is similar to that of AO for the alkaline earth metals. There is no direct evidence for that supposition at the present

time, but reaction (10) for La, Y and Sc cannot be discounted. Another possible chemi-ionization reaction which bears some resemblance to reaction (10) involves the trihydroxide A(OH)3: A(OH)3 + H ~ - e - + A(OH)3 H+ or A O + . 2 H 2 0 AH~I = IE°(H) - PA°(A(OH)3)

(11)

However, the occurrence of reaction (11) in these flames is rejected because the estimated concentration of the trihydroxide is very small. Thus, of the six reactions listed so far, only numbers (6), (8), (9) and (10) will be considered further. In addition, two possible chemical ionization (CI) reactions should be mentioned involving H30 +, small signals of which are detectable in all eight flames in the absence of metallic additives. For example, the largest signal is found in flame D where the H3 O+ profile rises near the reaction zone and then maintains a constant, small plateau value of 30 mV downstream. It is often believed that such signals stem from trace hydrocarbon impurities which undergo chemi-ionization [381 CH + O ~ HCO + + e -

(12)

followed by proton transfer to water HCO + + H20 ~,~ H3 O+ + CO

(13)

Our experience suggests that a different chemiionization reaction is largely responsible in these hydrogen flames [39] H + H + OH ~,~ H3 O+ + e-

(14)

because the measured signal (30 mV in flame D) is remarkably constant when many different cylinders of H2, O 2 and Ar have been used over a long period of time. Furthermore, no increase in signal is observed when a trace (< 10 -7 mole fraction) of hydrocarbon vapour is introduced. The flat plateau of the

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

H3 O+ profile suggests that reaction (14) is equilibrated. The oxide-hydroxide OAOH may be chemically ionized by proton transfer from H3 O+

65

1500 (a)

A

1000

Na* (with added CH4}

C

H3 O+ + OAOH ~ A(OH) + + H20 AH~5 = PA°(H20) - pA°(OAOH)

500

(15)

or the monoxide AO by electron transfer i

H3 O+ + AO ---+AO + + H20 + H

0

AH~6 = IE°(AO) - p a ° ( a o ) - 1E°(H)

i

i

10 Distance z

20

I

30

along flame axis [mm}

(16)

in analogy with the similar electron transfer reaction known to occur for the alkali metals [40]. Both reactions (15) and (16) should be exothermic, and it was not possible to distinguish between them. Normally, their contribution to the total metal ionization will be very minor. However, in those experiments described below where a small amount of CH4 was added to the flame gas for the express purpose of forming a high concentration of H3 O+, reactions (15) and/or (16) are of major importance.

4.3. Suggested mechanism for the flame ionization of group 3 metals The experimental basis for the proposed ionization mechanism is represented in Fig. 2. The particular case considered is that of flame D doped with La by spraying a 0.1 M aqueous solution of La(NO3)3.6H20 in the atomizer. It yields the total positive ion (TPI) profile for total La ions (LaO + and its hydrates) shown at the top of Fig. 2(a). The profile exhibits an initial rapid rise followed by a further, more gradual, steady increase downstream. The initial rise near the reaction zone (z = 0) is believed to stem from the chemiionization reaction (6) of La + O; this is the region where [La] could be large, coming from the decomposition of the salt crystallites and before LaO and OLaOH have had time to

1000

~>

(b)

800 600

~m ,n

additive}

400

200

(with

0

0

added

Na)

I

I

i

10

20

30

Distance z along flame axis (ram)

Fig. 2. Simulation of a La total positive ion (TP]) profile by thermal (collisional) ionization of Na plus chemical ionization of Na involving electron transfer to H3 O+, all in the fuel-rich flame D. The profiles in (a) were observed with the atomizer spraying a 0.1 M solution of La(NO3)3.6H20, or a 0.01 M solution of NaNO3, both with and without the addition of 0.5 cm 3 s -I of CH4; the latter produces H3 O+ by chemi-ionization. The actual H3 O+ profiles (including H30+.H20) from the added CH 4 are shown in (b) with Na both present and absent•

form, and [O] is enhanced by radical overshoot. Further downstream, where [La] is small but [LaO] becomes large, thermal ionization of LaO by reaction (9) takes over to explain the steady increase. To an unknown degree, chemi-ionization of OLaOH + H by reaction (10) may contribute. To illustrate the feasibility of this mechanism by analogy, the bottom profile in Fig. 2(a) shows just the thermal ionization of Na to form Na + by doping the same flame D with the atomizer spraying a 0.01 M NaNO3

66

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

solution; sodium was chosen because IE°(Na) = 5.139 eV [33], fairly close to that of LaO given in Table 2. The Na + profile exhibits the same steady increase downstream observed for the La ion profile, but not the initial rapid rise. To add an initial rise to the Na + profile, a small flow of CH 4 was added to the Na-doped flame. With sodium absent, the CH 4 addition produces the H3 O+ profile shown at the top of Fig. 2(b) by means of reactions (12) and (13). With sodium also present, the bottom profile in Fig. 2(b) is obtained; H3 O+ has decreased by reaction with Na to form Na + involving a charge transfer process akin to reaction (16) [40]. The shapes of the H 3 0 + profiles, which have an early peak, demonstrate that ions have been introduced which react early to provide an initial boost to the sodium ionization, resulting in the middle profile shown in Fig. 2(a). Its shape is very similar to the La ion profile given at the top. In summary, the group 3 metals in hydrogen flames appear to be ionized initially by chemi-ionization of atomic A with O near the flame reaction zone, but thermal (collisional) ionization of the monoxide AO becomes dominant further downstream. The chemi-ionization of OAOH with H is a possible but unknown contributor.

4.4. Total ionization profiles The temperature dependence of the lanthanum ionization is shown in Fig. 3(a) for the four F R flames and in Fig. 3(b) for the corresponding FL flames by means of total ionization profiles. All flames were doped with the atomizer spraying a 0.15 M aqueous solution of La(NO3)3.6H20, and the mass spectrometer was set to collect total positive ions, TPI. Not surprisingly, the ionization level increases with increasing temperature in all cases. When an ion profile is horizontal, it indicates an equilibrium situation where the rate of production equals the rate of loss.

1500

[a) Fuel-rich

E 1000

c

g B

500

A ,

0

10

I

I

20

30

Distance z along flame axis (mm)

750

E

500

E I-

250

(b) Fuel-lean

B' A'

I

;

I

I

0

10

20

30

Distance z along flame axis (ram)

Fig. 3. Temperature dependence in the range 1890 2425 K of the total positive ion (TPI) profile of La with the atomizer spraying a 0.15 M solution of La(NO3)3.6H20 in (a) the four fuelrich flames A - D , and (b) the four corresponding fuel-lean flames A'-D'.

The profiles in Fig. 3 show the relative importance of the initial chemi-ionization by La + O compared with thermal ionization of LaO and/or chemi-ionization by OLaOH + H further downstream. For the F R flames given in Fig. 3(a), chemi-ionization by reaction (6) in the hottest flame D raises the ionization level to a value less than the equilibrium level for thermal ionization by reaction (9) and/or chemi-ionization by reaction (10) such that the profile continues to rise downstream. Flames C and B appear approximately to attain equilibrium. In contrast, initial chemiionization in the coolest flame A raises the ionization level above its equilibrium value

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

for subsequent thermal ionization/chemiionization, and the profile decays back towards equilibrium downstream. Nevertheless, the downstream signal magnitudes at the four temperatures given in Table 1 exhibit an exponential dependence with inverse temperature, i.e. a plot of log(signal) versus lIT is approximately linear. Such a dependence is not unexpected for reaction (9) if the downstream signals are not too far from their equilibrium values. Exactly the same trend is observed for the four FL flames D ' - A ' at corresponding temperatures except that the signal magnitudes downstream are approximately half as large as those of their FR counterparts. These magnitudes show the same linear dependence for a plot of log(signal) versus lIT. For each pair of FR/FL flames at a given temperature, the total La concentration from the atomizer is the same. Moreover, the total fraction of gaseous La species should be the same in each pair if the gaseous species arise as an equilibrium "vapour pressure" of the solid La203; the vapour pressure depends only on the temperature and not on the flame composition. This is a general assumption for every metal in each pair of flames. The initial rate of ion production by reaction (6) will be higher in the FL flame than the FR one because [O]FL > [O]FR and the dependence on the disequilibrium parameter for radical overshoot is the same '72 factor in each. Thereafter, the rate of ion production by reaction (9) will be higher in the FR flame because [LaO]FR > [LaO]FL. If reaction (10) is operative, the F R rate will exceed the FL rate because [H]FR/[H]F L > 1 and, although [OLaOH]FR/[OLaOH]FL < 1, the product of the ratios is greater than 1. This might not be the case near the reaction zone as [H]FL has a "73 dependence compared with a first-power "7 dependence for [H]FR. Of course, at any point in a flame, the net rate of ion production is the production rate minus the loss rate. In that the

67

ions of the LaO+.nH20 series are closely coupled by reaction (8), ion loss in all flames will occur by the reverse reaction ( - 10) but not by (-6) which is endothermic. Undoubtedly other dissociative recombination reactions exist for the higher members of the hydrate series; presumably the three-body recombination reaction (-9) will be slower. The chemistry can be represented by a cyclic reaction scheme LaO + H20 (2)=OLaOH + H (9) T,[. M

Tl (10)

LaO + + e- + H20 ~ La(OH)~- + e(s) (17) such that reactions ( - 2 ) + (9) + (8) -- (10). The important point to note is that, if reactions (2) and (8) are balanced (designated by = sign), ion production by reactions (9) and (10) cannot be distinguished. An expression can be derived for the total positive ion signal [TPI] in terms of the total concentration of gaseous lanthanum [La]tot based on the following reasonable assumptions. (a) Reactions (2) and (8) are rapidly equilibrated. (b) The flame is a quasi-neutral plasma in which the concentration of negative ions is negligible, so that [TPI] = [e-] = [LaO +] + [La(OH)~-]; higher members of the LaO+.nH2 O hydrate series (n = 2, 3, ...), if present in the flame as genuine flame ions, are rapidly equilibrated with the lower members (n = 0, 1) through reaction (8). (c) For the gaseous species, [La]tot-[LaO] + [OLaOH]; the degree of ionization is small such that [TPI] < 1% of [La]tot. Under these conditions, the total positive ion signal is given by [TPI] = {[LaO](1 + Ks[H:O])

k9[M] + k 0K2[H20] ] 05 k_ 9 -]- k_ loK8 [H2O ] j

(18)

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

68

Table 4 Analysis of the signal magnitudes for total positive ions of lanthanum from reaction (2) and Eq. (18) Flame AG~--~ (kJmol 1) A A~ B Br C C~ D D~ a

([LaO]FR/[LaO]FL) 1/2 = [TPI]vR/[TP1]vI_

Fraction of [LaO] a 90

110

130

150

0.263 0.0138 0.504 0.0844 0.621 0.190 0.696 0.321

0.559 0.0472 0.757 0.220 0.823 0.400 0.860 0.561

0.818 0.149 0.905 0.463 0.930 0.655 0.943 0.775

0.941 0.384 0.967 0.726 0.974 0.844 0.978 0.903

90

110

130

150

4.37

3.44

2.34

1.57

2.44

1.85

1.40

1.15

1.81

1.43

1.19

1.07

1.47

1.24

l. 10

1.04

In every case, (fraction of [OLaOH]) = 1 - (fraction of [LaO]).

where K2 and K8 are equilibrium constants. In terms of [La]tot, [LaO]=[La]tot[H]/ ([HI + K2[H20]). For a pair of F R / F L flames at a given temperature, Eq. (18) shows that [TPI] oc [LaO]°5; the rest of the expression amounts to a constant, as [H20 ] is nearly constant (as shown in Table 1). For the four pairs of flames, the experimental observation that the ratio [TPI]FR/[TPI]F L is ~2--4 imposes a fairly severe constraint on the value of AG~=-RTlnK2, as [OLaOH]/[LaO 1 = K2 [H20]/[HI. Reaction (2) amounts to the dissociation of H - O H followed by formation of the O L a - O H bond. The calculated data in Table 4 based on Eq. (18) show how the ratio [TPI]vR/[TPI]vcvaries with AG~ in the range 90-150 kJ mo1-1. In this restricted range, the fraction of total lanthanum present in a flame as LaO varies from less than 2% in flame A t to more than 97% in flame D; correspondingly, the fraction of OLaOH varies from > 98% to < 3%. The assumption has been made that [La]tot is the same for both flames in each pair. Before going any further, it should be recognized that the production of ions by thermal ionization, reaction (9), and presumably also by endothermic chemi-ionization, reaction (10), will have a strong positive temperature dependence. Consider an activation energy

for reaction (9) equal to the reaction endothermicity, IE°(LaO) = 4.9 eV; this has been shown to be true for the thermal ionization of the alkali metals [41]. It is then possible to calculate the temperature increase A T necessary to increase the equilibrium ionization level by, say, a factor of 2. The value of AT for a factor of 2 ranges from 44 K for flames A/ A' at 1900 K up to 72 K for D/D' at 2425 K. Evidently a small temperature change can produce a large variation in the TPI signal. This realization is undoubtedly a consideration in our observation that the ratio [TPI]vR/[TPI]FL for a given pair of flames varies slightly from one day to the next, generally in the range 2-4. This borders on the accuracy with which the flowmeter settings can be reproduced for supplying the hydrogen and/or oxygen flows to the burner. Possibly a more important consideration has been mentioned already. Although both flames in each pair have nominally the same adiabatic flame temperature (see Table 1), the F R flame burns a little hotter due to afterburning and the FL flame a little cooler due to air entrainment. Therefore this temperature dependence must be kept in mind in choosing a "best" AG~ from Table 4; values of the ratio [TPI]F~/[TPI]Fc closer to unity should be favoured. With all of these considerations in mind, a

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

value chosen from the middle of Table 4 appears to be the most reasonable; namely, AG~ = 120 4-30 kJ mo1-1, in which a fairly large uncertainty has been included. A higher value overemphasizes [LaO], thereby precluding rapid proton transfer to OLaOH. A lower value moves outside the measured range of [TPI]FR/[TPI]F L. For reaction (2), a rough value of TAS~ has been estimated from other species in the +3 oxidation state for which thermodynamic data are available [10]. In the temperature range 1900-2400 K, TAS~ varies from - 3 2 to - 3 9 kJ mo1-1 for aluminium and from - 4 0 to - 4 6 kJ mo1-1 for boron; higher mass favours the smaller absolute value. Taking TAS~ = - 3 5 + 10 kJ mo1-1, AH~ _~ D ~ ( H - O H ) - D ~ ( O L a - O H ) = 85 -t- 40 kJ mo1-1 . Using D~(H-OH) = 493.3 kJ mol- 1 [ 10], a value for D~ (OLa - H ) = 408 + 40 kJ mo1-1 (97.6 4- 9.6 kcal mol-1; 4.23 4- 0.41 eV) is obtained. Fortuitously, this value is remarkably close to our original estimate for the bond dissociation energy of O L a - O H , equivalent to D~(OLa O) = 4.20 + 0.33 eV (405 + 32 kJ mo1-1) from Clemmer et al. [6]. The same general behaviour described for La was also observed for the other group 3 metals Y and Sc. The total ionization profiles of the three metals are compared in the pair of hot flames near 2425 K, for the F R flame D in Fig. 4(a) and the FL flame D' in Fig. 4(b). In all cases, the metal was introduced with the atomizer spraying a 0.1 M aqueous solution of the metallic salt, slightly lower than the 0.15 M concentration used for the profiles of Fig. 3. For each metal, the same F R / F L factor of 2 - 4 is apparent for the signal magnitudes. The profile shapes for all three metals in both flames show an initial rise due to chemiionization followed by a slower increase towards the equilibrium level for thermal ionization. For each flame, the downstream signal magnitudes of the three metals show an approximate exponential dependence on the ionization energies of their monoxides

1500

'

i

•

T

69

,

i

(a) Fuel-rich

E

1000

g E

500

Y

Sc

0

i

i

i

10

20

30

Distance z along flame axis (ram) 600 (b)

E

Fuel-lean

400

E

E

200

0

10

20

30

Distance z along flame axis [ram)

Fig. 4. Total positive ion (TPI) profiles of the group 3 metals La, Y and Sc measured with the atomizer spraying 0.1 M aqueous solutions of their respective salts near 2425 K in (a) fuel-rich flame D, and (b) fuel-lean flame D'.

given in Table 2; i.e. a plot of log(signal) versus - I E ° ( A O ) / R T is linear. Such a dependence might be expected for a thermal ionization process. The results in Fig. 5 show the effect on the total ionization profiles of La, Y and Sc when chemical ionization (CI) is produced by the presence of H30 + as a proton transfer or electron transfer reagent. In each flame, the same flow of 0.5 cm 3 s -1 of C H 4 was added to the unburnt gas to give H 3 0 + via reactions (12) and (13); the H3 O+ profile shape with no additive metal is given in Fig. 2(b) for flame D. The metals were added by spraying 0.1 M salt solutions in the atomizer. Total ion profiles with added C H 4 are given for La, Y and Sc,

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

70

2000 (a) Fuel-rich

1500

1000 a_

Sc

500

L

I

I

10

20

30

Distance z along flame axis (mm) 2000

j-

(b) Fuel-lean

>

1500

E ~ E

La

La, the TPI signal downstream was 1300 mV in B compared with 1200 mV in B'. This essential equivalence of the TPI signals in each pair of flames with added CH 4 is a natural consequence of the ion chemistry. The [H3 O+] formed by reactions (12) and (13) depends primarily on the added [CH4] and the temperature, and should be the same for each pair of flames. In fact, the measured peak H3 O+ signal tends to be slightly higher in the F R member of the pair, presumably because the F R flame is slightly hotter. Proton and/or electron transfer to the metallic species then proceed at high rates, but the fundamental relationship of the two processes can be perceived from the following reaction sequence

1000

H3 O+ + OAOH ~ A(OH)~ + H20

(15)

A(OH) + + M ~ AO + + H20 + M

(-8)

0_

f-

500 Sc

AO + H 2 0 ~ OAOH + H 0

10

20

(2)

30

Distance z along flame axis (mm} Fig. 5. Total positive ion (TPI) profiles o f L a , Y and Sc under the same conditions as those for Fig. 4 except that 0.5 cm 3 s -~ of CH 4 has been added to introduce H3 O+ as a chemical ionization (CI) agent, in (a) fuel-rich flame D, and (b) fuel-lean flame D'.

for the F R flame D in Fig. 5(a) and for the FL flame D' in Fig. 5(b). They represent the sum of H3 O+ and metal ions, although H3 O+ is effectively consumed beyond z = 10 m m downstream. The profiles in the region z = 10-30 m m may be compared directly with those of Fig. 4(a) and (b). In Fig. 5(a), the downstream signal magnitudes for all three metals have increased slightly. In Fig. 5(b), however, the increases are substantial (a factor of 3-5) such that the ionization levels for each metal appear to be nearly equal in this pair of F R and FL flames at the same temperature. Exactly the same result was achieved in other pairs of flames. For example, with constant CH 4 add#d to flames B and B' doped with

H3 O+ + A O ~ - A O + + H20 + H

(16)

such that reaction (16) = (15) + ( - 8 ) + (2). Assuming that reactions (2) and (8) are balanced as before, this means that reactions (15) and (16) cannot be distinguished. Moreover, the rate coefficients at flame temperatures for proton transfer reactions like (15) and for electron transfer reactions like (16) for the alkali metals are all large, with values close to 10-s cm 3 molecule s -1 per molecule which are not strongly temperature dependent [42]. This means that the rate for the production of metallic ions near the flame reaction zone by reactions (15) and/or (16) is considerably faster than the production rate by chemi-ionization, reactions (6) and (10), or thermal ionization, reaction (9). In that the total gaseous metallic species [A]tot = [AO] nt- [OAOH], the growth of total metallic positive ions TPI is given by

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

[H30+] (k15[OAOH] +k16[AO]) = k[H30+] {A~], with k = k15 --~ k16, i.e. it depends on the total concentration of gaseous metallic species, which is essentially the same in both the FR and FL flames of each pair. The TPI signal levels off when effectively all of the [H3O+] has reacted away; more precisely, [H3 O+] drops to a very small value determined by the equilibrium constant Kl5 o r K16. The decay of [H3 O+] with metal present is similar to that shown in Fig. 2(b) for sodium. The important point is that the TPI signal does not depend on the relative amounts of [OAOH] and [AO]. The initial production of metallic ions by proton and/or electron transfer raises the TPI signal above its equilibrium level determined by reactions (9) and (10), and the signal subsequently decays by electron-ion recombination back towards the level given by Eq. (18). From Fig. 5, the decay rate is rapid for Y and Sc, but less so for La because its equilibrium level is higher.

4.5. Individual ion profiles In Fig. 6, profiles are shown for all of the individual La ions which were observed in the pair of hot flames near 2425 K: (a) in the FR flame D in the left column and (b) in the FL flame D t in the right column. In both cases, the largest signal was observed for the first hydrate LaO+.H2 O, corresponding to La(OH)2 +. For flames at lower temperature such as A and A t near 1900 K (not shown), a fifth ion signal is observed corresponding to LaO+.4H2 O but its relative magnitude is much smaller than that of the preceding hydrate LaO+.3H2 O. A simple explanation is that the third hydrate corresponding to La(OH)z(H20)2- completes an electron octet around the central La atom and the fourth hydrate lies outside the coordination sphere. For all of the group 3 metals, the only ions observed were AO + and its hydrates. Signals corresponding to other

71

possible ions such as A +, AOH + and AO + were never found. With added C H 4 to promote additional chemical ionization (CI) by proton or electron transfer (not shown), all of the profile signal magnitudes increased but the relative order amongst the various hydrates did not change. Presumably an ion pool is involved in which the various La ions are rapidly equilibrated amongst themselves by the generalized hydration reaction (8); the reaction will proceed at a high rate in both directions in the burnt gas because water is a major product (to the right), and because the third-body concentration [M] is large at atmospheric pressure and the efficiency is high at the high flame temperature (to the left). Near the flame reaction zone close to z = 0, the rate of rise of the profile is more rapid the higher the hydrate; the highest hydrate often exhibits an initial sharp peak. In this region, the temperature profile has not yet reached its maximum value, and higher hydrates are relatively enhanced by the lower temperature. However, another temperature effect is evident in Fig. 6. The temperature profile along the flame axis is normally assumed to rise rapidly in the reaction zone and then bend over to give a relatively constant isothermal plateau throughout the burnt gas. This will be true for the burners used to measure temperatures by sodium D line reversal, where sodium is added to the central portion of a flame which is surrounded by a shield flame of the same composition without added sodium [43]. For the present unshielded burner of diameter 10.5 mm, an isothermal burnt-gas region is generally assumed for a distance z of about 2.5 burner diameters when sampling on the flame axis. However, it is apparent from Fig. 6 that the temperature continues to rise downstream through the burnt gas in FR flame D because of afterburning; i.e. the slopes of the profiles downstream decrease with increasing mass number such that the lower hydrates grow at the expense of the higher

72

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

(a) Fuel-rich

(b) Fuel-lean 200

250 200

150

150 100

M155 +

100 50

LaO+

50 0

i

I

I

LaO +

I

I

600

i

300

'

2O0 300 100

/

1500

' ~

I

LaO+'HaO ,

,

I

,

0

/

I

300 , ,

LaO*.H20 .

i

i

I

, 100

_8

50

100

/ 0

25 [ /

LaO*.2H20

II

I

I

0

/I

LaO+.2H20

I

I

I

15

50

M209 +

40

LaO+.3H=O

10 30

LaO÷.3HaO

10 0

._jl j f

M209 +

20

i

i

I

10

20

30

0

I

i

i

10

20

30

Distance z along flame axis (mm) Fig. 6. Individual profiles for all of the La ions observed with the atomizer spraying a 0.1 M solution of La(NO3)3•6H20 in (a) fuel-rich flame D, and (b) fuel-lean flame D'.

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55-76

ones, indicative of a temperature increase. In contrast, the temperature falls downstream in the coned-in FL flame D' owing to air entrainment; i.e. the slopes of the profiles downstream increase with increasing mass number such that the higher hydrates grow at the expense of the lower ones, indicative of a temperature decrease. The magnitude of the rise or fall in temperature in the downstream "plateau" region is of the order of 100 K. This is a general phenomenon which has been consistently observed in other pairs of F R / F L flames at the same nominal temperature. Evidently the flame diameter is not large enough for adequate shielding of the flame axis by its perimeter. However, larger flames are undesirable because of the increased requirements for both heat dissipation and gas supplies to the burner. Profiles are given for all of the individual ions observed in F R flame D for Y in Fig. 7(a) and for Sc in Fig. 7(b) with the atomizer spraying a 0.1 M aqueous solution of the appropriate salt. In most respects, the profiles are similar to those observed for La. Differences are more in the nature of trends through the group 3 metals rather then new phenomena. For example, the nude AO + ion was observed only at the limit of detection (~ 0.1 mV) with Y or Sc, unlike La. This is not interpreted to mean that the ionization occurs by reactions other than (6), (9) and (10). Rather, it implies that the rate of conversion of AO + to AO+.H2 O (equivalent to A(OH)~-) by reaction (8) is very rapid for Y and Sc. Not surprisingly, the signal magnitudes decrease in the order La, Y, Sc, in line with the increasing ionization energies of their monoxides AO, as explained in connection with Fig. 4. Interestingly, the maxim u m signals amongst the individual hydrate ions of each metal form a progression: L a O + . H 2 0 , Y O + . 2 H 2 0 and SCO+-3H2 O in F R flame D, but the same progression is observed in FL flame D' (not shown).

73

Simplistically, it might be said that the octet rule is more rigorously obeyed the smaller the metal atom and the coordination bonds of water are stronger. Although the A O + . 4 H 2 0 ions were observed with Y and Sc (but not with La) with the implication that one water molecule lies outside the coordination sphere of the central atom, the signal magnitudes of the fourth hydrates are much smaller than those of AO +.3H20, which is equivalent to A(OH)z(H20)2 ~. Other differences of Y and Sc compared with La are minor.

5. Summary and conclusions Four pairs of premixed, fuel-rich/fuel-lean (FR/FL; equivalence ratio 0 = 1.5/0.75), H 2 O2-Ar flames at four temperatures in the range 1900-2425 K, all at atmospheric pressure, were doped with ~ 10-6 mole fraction of each of the group 3 metals La, Y and Sc using atomizer techniques. Metallic ions produced in the flames were observed by sampling a given flame through a nozzle into a mass spectrometer. Profiles of the ion concentration (individual, or total positive ions TPI) are measured as a function of distance (or time) along the flame axis. From these studies, a number of general hypotheses can be summarized and conclusions drawn. (1) An unknown fraction of the total group 3 metal exists in a flame as the sesquioxide AzO3(s) (where A is the metal atom) in decreasing order La > Y > Sc. Initially, a concentration [A] of free metal atoms exists near the flame reaction zone, but it disappears further downstream. At thermal equilibrium downstream, the total gaseous metal is present only as the monoxide AO and the oxidehydroxide OAOH. Less than 1% of the gaseous metal is present as ions. At a given temperature, the total concentration of gaseous metal is independent of the flame composition but [AO] is favoured in F R

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

74

(a) Y t t r i u m

ions

(b) S c a n d i u m

50

10

40

8

ions

6

20

M123"

4

M79*

10

YO+'H20

2

ScO+.H20

0

I

0

i

I

200

I

100

150

75

f

100 M97"

M141 +

E

50

25

YO*.2H20

ScO+.2H=O

m ,

0

I

e-

150

125

,

,

,

= m

¢.-

0 M159 + 5O

50

M115"

/

0

r

,

J

J

,

12

,

-

,

,

-

-a

0

25

Sc0".4H20 15

6 10

0

0 r 0

10

20

30

0

10

20

30

Distance z along flame axis (mm} Fig. 7. Individual profiles for all of the metal ions observed in fuel-rich flame D with the atomizer spraying a 0.1 M aqueous solution of (a) YC13 .6H20, and (b) SeCI 3.

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

flames; [OAOH] is higher in FL flames because [H] is lower. (2) For each metal, the only metallic ions observed can be represented by an oxide ion series A O + . n H 2 0 (n = 0-3 or more). The higher hydrates are enhanced by the cooling which occurs at the nozzle during sampling, an artefact of the sampling process. However, some of the lower members of the series represent true flame ions which are not simple hydrates, e.g. A(OH)~- for n = 1, and the protonated trihydroxide A(OH)3 H+ for n -- 2. The signal magnitudes of an ion series drop off abruptly with the completion of an electron octet around the metal atom by two coordinated water molecules, A(OH)z(H20)~- for n = 3. (3) Higher n values are favoured as the flame temperature is lowered, and for lower mass of the metal atom ( L a > Y > S c ) at a given temperature. A similar distribution of relative signal magnitudes with n is observed for F R and FL flames at the same temperature; however, the TPIvR signal always exceeds the TPIFL signal by a factor of 2-4. (4) With regard to the mechanism of ion production, the ionization receives an initial boost near the flame reaction zone where [A] may be large by the chemi-ionization reaction of A with atomic O to form AO +, followed by rapid three-body association reactions with water to form the equilibrated ion series AO+.nH2 O. Further downstream, the ionization level is sustained by the thermal (collisional) ionization of AO and/or the chemi-ionization of OAOH with H; the two reactions cannot be distinguished. Ions are lost by dissociative electron-ion recombination of A(OH)~-, and perhaps higher members of the ion series as well. (5) For a given metal in the four F R or FL flames, the TPI signal exhibits an exponential increase with increasing temperature. This strong positive temperature dependence means that the TPI signal can double for a temperature increase A T < 100 K. Also, for

75

different metals (La, Y and Sc) in a given flame, the TPI signal shows an inverse exponential dependence on IE°(AO), the ionization energy of the monoxide. The magnitude of the TPI signal varies as [AO] °5. (6) An estimate was obtained for the bond dissociation energy of lanthanum oxide hydroxide D~(OLa-OH) = 408 + 40 kJ mol -l . (7) The same small amount of CH 4 was added to the flames to promote chemical ionization (CI) of metallic species by H3 O+ as a CI reagent. The reactions of H3 O+ involve proton transfer to OAOH to produce A(OH)~and/or electron transfer to AO to produce AO+; the two reactions have similar rate coefficients and cannot be distinguished. For a pair of F R / F L flames at the same temperature doped with the same quantity of metal, the ionization level attained was the same. This level depends only on [H30+], the temperature and [A]tot, the total concentration of gaseous metal, and not on the concentration ratio [OAOH]/[AO]. CI raises the ionization level above the equilibrium ionization level for thermal and/or chemi-ionization. The superequilibrium CI level subsequently decays by enhanced electron-ion recombination. (8) The shapes of the individual ion profiles confirm an important fact about a pair of FR/ FL flames having nominally the same adiabatic flame temperature. The temperature of the FR flame increases slightly downstream because of afterburning, whereas that of the FL flame decreases slightly through air entrainment. For different metals in a given flame, the maximum ion signal is obtained for a higher member of the AO + .nH20 series when the atom is smaller; e.g. LaO+.H2 O, YO+.2H20 and SCO+.3H2 O. Evidently the coordination tendency to fill the electron octet is greater for the smaller atom.

Acknowledgement Support of this work by the Natural

76

P.M. Patterson, J.M. Goodings/International Journal of Mass Spectrometry and Ion Processes 148 (1995) 55 76

Sciences and Engineering Research Council of Canada is acknowledged.

References [1] M.C.R. Cockett, J.M. Dyke, A.M. Ellis, M. Feh~r and A. Morris, J. Am. Chem. Soc., 111 (1989) 5994. [2] M.C.R. Cockett, J.M. Dyke, A.M. Ellis, M. Feh6r and T.G. Wright, J. Electron Spectrosc. Relat. Phenom., 51 (1990) 529. [3] M.C.R. Cockett, J.M. Dyke, A.M. Ellis and T.G. Wright, J. Chem. Soc., Faraday Trans. 1, 87 (1991) 19. [4] M.C.R. Cockett, L. Nyul/Lszi, T. Veszpr+mi, T.G. Wright and J.M. Dyke, J. Electron Spectrosc. Relat. Phenom., 57 (1991) 373. [5] J.M. Dyke, A.M. Shaw and T.G. Wright, in A. Fontijn (Ed.), Gas-Phase Metal Reactions, North-Holland, Amsterdam, 1992, p. 467. [6] D.E. Clemmer, N.F. Dalleska and P.B. Armentrout, Chem. Phys. Lett., 190 (1992) 259. [7] R. Kelly and P.J. Padley, Trans. Faraday Soc., 65 (1969) 367. [8] J.M. Goodings, C.S. Hassanali, P.M. Patterson and C. Chow, Int. J. Mass Spectrom. Ion Processes, 132 (1994) 83. [9] J.M. Goodings, Q. Tran and N.S. Karellas, Can. J. Chem., 66 (1988) 2219. [10] M.W. Chase, C.A. Davies, J.R. Downey, D.J. Frurip, R.A. MacDonald and A.N.Syverud, JANAF Thermochemical Tables, 3rd edn., J. Phys. Chem. Ref. Data, 14 (Suppl. 1) (1985). [11] J.M. Goodings, S.M. Graham and W.J. Megaw, J. Aerosol Sci., 14 (1983) 679. [12] A.N. Hayhurst and D.B. Kittelson, Combust. Flame, 28 (1977) 137. [13] A.N. Hayhurst and N.R. Telford, Proc. R. Soc. London, Ser. A, 322 (1971) 483. [14] N.A. Burdett and A.N. Hayhurst, J. Chem. Soc., Faraday Trans. 1, 78 (1982) 2997. [15] P.F. Knewstubb and T.M. Sugden, Proc. R. Soc. London, Ser. A, 255 (1960) 520. [16] J.M. Goodings, P.M. Patterson and A.N. Hayhurst, J. Chem. Soc., Faraday Trans. 1, in press. [17] R.M. Fristrom and A.A. Westenberg, Flame Structure, McGraw-Hill, New York, 1965, p. 330. [18] D.R. Jenkins and T.M. Sugden, in J.A. Dean and T.C. Rains (Eds.), Flame Emission and Atomic Absorption Spectrometry, Vol. 1, Marcel Dekker, London, 1969, p. 151. [19] P.J. Padley and T.M. Sugden, Seventh Symposium (International) on Combustion, Butterworths Scientifc Publications, London, 1959, p. 235.

[20] E.M. Bulewicz, C.G. James and T.M. Sugden, Proc. R. Soc. London, Ser. A, 235 (1956) 89. [21] G.L. Schott, J. Chem. Phys., 32 (1960) 710. [22] W.E. Kaskan and G . L Schott, Combust. Flame, 6 (1962) 73. [23] P.J. Padley, F.M. Page and T.M. Sugden, Trans. Faraday Soc., 57 (1961) 1552. [24] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 17 (Suppl. 1) (1988). [25] J.B. Pedley and E.M. Marshall, J. Phys. Chem. Ref. Data, 12 (1983) 967. [26] I. Barin, Thermochemical Data of Pure Substances, 2nd edn., VCH, Weinheim, 1993. [27] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney and R.L. Nutall, J. Phys. Chem. Ref. Data, 11 (Suppl. 2) (1982). [28] K.P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [29] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 73rd edn., CRC Press, Boca Raton, FL, 1992. [30] P.N. Walsh, H.W. Goldstein and D. White, J. Am. Ceram. Soc., 43 (1960) 229. [31] R.J. Ackermann and E.G. Rauh, J. Chem. Thermodyn., 3 (1971) 445. [32] B.F. Yudin, V.I. Mogilenskii, Yu.A. Polonskii and S.A. Lapshin, Zh. Prikl. Khim., 49 (1974) 776. [33] C.E. Moore, Ionization Potentials and Ionization Limits Derived from the Analysis of Optical Spectra, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. (U.S.), No. 34, 1970. [34] S.G. Lias, J.F. Liebman and R.D. Levin, J. Phys. Chem. Ref. Data, 13 (1984) 695, and supplements available from the authors. [35] K. Schofield and T.M. Sugden, Tenth Symposium (International) on Combustion, Academic Press, New York, 1965, p. 589. [36] A.N. Hayhurst and D.B. Kittelson, Proc. R. Soc. London, Ser. A, 338 (1974) 175. [37] E. Murad, J. Chem. Phys., 75 (1981) 4080. [38] J.A. Green and T.M. Sugden, Ninth Symposium (International) on Combustion, Academic Press, New York, 1963, p. 607. [39] A.N. Hayhurst and N.R. Telford, J. Chem. Soc., Faraday Trans. 1, 71 (1975) 1352. [40] A.N. Hayhurst and N.R. Telford, Trans. Faraday Soc., 66 (1970) 2784. [41] A.F. Ashton and A.N. Hayhurst, Combust. Flame, 21 (1973) 69. [42] D.E. Jensen and G.A. Jones, Combust. Flame, 32 (1978) 1. [43] A.G. Gaydon and H.G. Wolfhard, in Flames, Their Structure, Radiation and Temperature, 3rd edn., Chapman and Hall, London, 1970, p. 240.