The Determination of Isotopically Labeled Species by FT-IR Difference Spectroscopy in the Study of Catalytic Reaction Mechanisms

The use of FT-IR gas-phase difference spectroscopy for the determination of isotopically labeled species in the study of catalytic reaction mechanisms is illustrated. Applications are based on the study of the mechanism of oxidative coupling of methane over oxide catalysts and include determinations of: (1) deuterated and partially deuterated ethylenes and ethanes from experiments with 2H labeling of the methane; (2) 18O contents of CO and CO2from experiments with 18O labeling of O2, CO, and CO2; (3) 13C/12C ratios in hydrocarbons and carbon oxides from experiments with 13C labeling of C2hydrocarbons. The major limitation of the technique is in regions of strong absorption where nonlinear effects make subtraction difficult; however, this limitation could be alleviated by judicious choice of pathlength. This technique is a powerful supplement to measurements based on mass spectrometry.