Ion chemistry of transition metals in hydrocarbon flames. I. Cations of Fe, Co, Ni, Cu, and Zn

Abstract

A fuel-rich, premixed, conical, methane-oxygen flame at 2200 K and atmospheric pressure is doped with approximately 1 ppm of the transition metals Fe, Co, Ni, Cu, and Zn. Metallic ions of these metals and their compounds formed by chemical ionization reactions with H 3 0 + are observed by sampling the flame through a nozzle into a quadrupole mass spectrometer. Concentration profiles of individual and total cations are measured as a function of distance along the flame axis, and also mass spectra at a fixed point in the burnt gas. For a given metal A, the mass spectra are dominated by the atomic ion A+ with smaller amounts of the molecular ions AH+, AOH', A(OH)H+, A(oH)~H+, and ACO+ and their hydrates. The spectra for Fe, Co, Ni, and Cu are very similar, but no ions are observed for Zn. The ion chemistry is dominated by proton transfer reactions from H 3 0 + to A and to the metallic compounds AO, AOH, and A(OH)2 which exist in the flame. In addition, A+ can be formed from the reaction of H 3 0 + with A by a charge transfer process. Also, some ions are formed by three-body association and free radical stripping reactions. The chemistry is discussed in detail to explain the relative magnitudes of the ion signals observed. In particular, when the atomic A+ ion is dominant, its concentration can reach a superequilibrium level early in the burnt-gas region before it slowly decays downstream; the phenomenon is similar to the free radical overshoot which occurs in hydrogen flames. [Traduit par la revue] Introduction We recently had occasion to consider the ion chemistry of alkali and alkaline earth metals in a premixed, fuel-rich, CH4-C2H2-O2 flame (I). This arose in the context of soot suppression by metallic additives involving an ionic mechanism (2, 3). We have now extended these studies to include chemical ionization (CI) processes for the first row of ten transition metals, Sc to Zn (atomic numbers 2 1-30). The last five including Fe, Co, Ni, Cu, and Zn are described here; the first five including Sc, Ti, V, Cr, and Mn present some different features, and are the subject of a second paper.A premixed, fuel-rich, CH4-O2 flame doped with metallic additive is sampled into a mass spectrometer to ascertain what metallic cations are present and where in the flame they occur. The interest in the problem stems from the considerable variety of oxidation states known for these metals, and the bond formation and structural configurations which they may exhibit at high temperature.Basically two mass-spectrometric flame studies involving cations of some of these transition metals have appeared in the past. In hydrogen flames, Hayhurst and Telford (4) observed ions for Cr, Mn, Fe, and Cu, and also other metals, in the course of measuring rate constants attributed to the charge transfer reaction

Periodic Trends Manifested through Gas-Phase Generation of Anions Such as [AlH4]−, [GaH4]−, [InH4]−, [SrH3]−, [BaH3]−, [Ba(0)(η2-O2CH)1]−, [Pb(0)H]−, [Bi(I)H2]−, and Bi– from Formates

“…10,22−24 Mass spectra of a number of diatomic transition-metal hydrides have been recorded under field evaporation 25 by Cs-beam sputtering on metal surfaces while spraying hydrogen or ammonia 26 or by spraying an aqueous solution of the metallic chloride into a 2200 K methane–oxygen flame. 27 Extensive investigations have been carried out with gas-phase hydride species 28 generated from silicon, 29 boron, 30,31 aluminum, 31,32 silver, 33−38 copper, 35,36,39,40 magnesium, 41 vanadium, 42 chromium, 42 iron, 42 cobalt, 42 and molybdenum; 42 however, there is still a paucity of information about hydrides of many other metals.…”

“…In general, the gas-phase generation and analysis of complex hydride anions have been accomplished only under very rigorous experimental conditions. For example, Al m H n – -type clusters have been generated in a discharge between an anode and a grounded aluminum sample cathode engulfed with hydrogen gas at 200 psi. ,− Mass spectra of a number of diatomic transition-metal hydrides have been recorded under field evaporation by Cs-beam sputtering on metal surfaces while spraying hydrogen or ammonia or by spraying an aqueous solution of the metallic chloride into a 2200 K methane–oxygen flame . Extensive investigations have been carried out with gas-phase hydride species generated from silicon, boron, , aluminum, , silver, − copper, ,,, magnesium, vanadium, chromium, iron, cobalt, and molybdenum; however, there is still a paucity of information about hydrides of many other metals.…”

Ion chemistry of second-row transition metals in hydrocarbon flames: cations and anions of Y, Zr, Nb, and Mo

“…A few years ago, we studied the cation chemistry of the first row of 10 transition metals Sc-Zn in a premixed, fuel-rich, CH4-0, flame (1,2). The metals were introduced at a concentration of mol fraction, and ions were produced by chemical ionization (CI) of metallic flame species by H30f, which occurs naturally in any hydrocarbon flame.…”

“…Apart from atomic cations A+, metallic flame ions exhibit metal-oxygen bonds almost exclusively, although AHf has been observed for Mn (2), Fe, Co, and Ni (1). For the second-row transition metals, Armentrout and co-workers (4,5) have measured the bond dissociation energies D(Af -H) to be 25954 (Y), 230+ 13 (Zr), 2262 13 (Nb), and 1765 13 (Mo) kcal mol-'.…”

Ion chemistry of aluminum and boron in methane–oxygen flames

“…The motivation for this work on aluminum and boron arose out of our previous studies of the ion chemistry of the first row of 10 transition metals (Sc-Zn) in a hydrocarbon flame (1,2). In these studies, the nude metallic oxide ion was obsewed for the first time in flames (2); i.e., AO', where A is the metal atom.…”