Chemical kinetics of lanthanum ionization in H 2–O 2–N 2 flames

Abstract

Trace amounts (≤10 −6 mol fraction) of lanthanum were introduced into five, premixed, fuel-rich, H 2–O 2–N 2 flames at atmospheric pressure in the temperature range 1820–2400 K. Aqueous salt solutions of the metal were sprayed into the premixed flame gas as an aerosol using an atomizer technique. The concentrations of the major neutral species present in flames, believed to be LaO and OLaOH, are linked by the balanced reaction LaO + H 2O = OLaOH + H having an equilibrium constant K = 0.1859 exp(−11 464/ T), based on a crude estimate of the bond dissociation energy for lanthanum oxide-hydroxide D 0 0(OLa–OH) = 408 ± 40 kJ mol −1. Metallic ions were observed by sampling the flames through a nozzle into a mass spectrometer and, for kinetics purposes, were measured as profiles of ion concentration versus distance (i.e. time) along the flame axis. The ions detected can be represented by an oxide ion series LaO +. nH 2O ( n = 0−3 or more); the genuine flame ions are thought to be LaO + ( n = 0) and La(OH) 2 + ( n = 1), equivalent to protonated OLaOH. The two ions are linked by the fast balanced reaction LaO + + H 2O + M = La(OH) 2 + + M, where M is a third body. The major ion production processes appear to be the chem-ionization reaction OLaOH + H ⇄ La(OH) 2 + + e −, and thermal (collisional) ionization LaO + M ⇄ LaO + + e − + M because the ionization energy IE 0 0(LaO) = 4.90 eV is low. Ion loss processes involve dissociative electron–ion recombination. When electron-ion recombination of LaO +. nH 2O was made dominant by the trace addition of potassium, values of the global recombination coefficient are given by (1718 ± 515) T −3.0±0.2 cm 3 molecule −1 s −1. The rate constant for thermal (collisional) ionization of LaO was found to be 8.0 × 10 −10 T 1/2 exp(−IE 0 0/RT) cm 3 molecule −1 s −1. The pre-exponential factor or cross section is only 8% of that determined for the alkali metals. The rate constant for the chemi-ionization of OLaOH is given by (5.6 ± 1.7) × 10 −12 exp(−17 350/ T) cm 3 molecule −1 s −1. Although no direct evidence was found for particle formation in these flames, both rate constants for ion production would be lower limits if appreciable lanthanum were present as solid particles.