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 CO₂ from experiments with ¹⁸O labeling of O₂, CO, and CO₂; (3) ¹³C/¹²C ratios in hydrocarbons and carbon oxides from experiments with ¹³C labeling of C₂ hydrocarbons. 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.