Metallic ions in hydrocarbon flames. I. Ion chemistry involving proton transfer reactions

Abstract

Small amounts ( < 10 −6 mol fraction) of alkali and alkaline earth metals are added to a conical, premixed, fuel-rich, CH 4-C 2H 2-O 2 flame at atmospheric pressure. Ion concentration profiles are measured as a function of distance along the flame axis by sampling the flame into a mass spectrometer. Metallic ion profiles for all of the metals include A + and AOH 2 + (= A +·H 2O) where A is the metal atom and, in addition, for the alkaline earths, AOH + and A(OH) 2H + (= AOH +·H 2O). The metallic ion signals are made comparable with, or greater than, those of the natural C x H y O z H + ions present in any hydrocarbon flame whose ion chemistry is dominated by proton transfer reactions involving C x H y O z neutral flame intermediates. Apart from A +, the metallic ions can be interpreted as protonated forms of the metallic neutral compounds AOH, AO and A(OH) 2 which are present in the flame in appreciable amounts for most of the metals. Moreover, most of the metallic compounds have proton affinities higher than those of the natural C x H y O z flame intermediates. Experimental observations and a discussion of the ion chemistry are presented to show that proton transfer reactions can be expected to dominate the metal ion chemistry in the reaction zone and the early downstream region of a hydrocarbon flame.