Millisecond FT-IR Spectroscopy of Surface Intermediates of C2H4 Hydrogenation over Pt/Al2O3 Catalyst under Reaction Conditions

Time-resolved rapid-scan FT-IR spectra of ethylene hydrogenation over alumina-supported Pt catalyst at 473 K were recorded under continuous H2/N2 flow (1 atm) and pulsed release of C2H4 (30 millisecond duration). Two surface species were observed, namely ethylidyne (CH3CPt3) with peaks at 2880 and 1339 cm-1 (lifetime 300 ± 50 ms), and a substantially shorter-lived intermediate with an intense band at 1200 cm-1 and weak absorptions in the 2875−2860 cm-1 region (lifetime around 100 ms). Comparison of the C2H4 + H2 results with those of experiments using D2 or C2D4 suggests that the 1200 cm-1 species is a surface ethyl intermediate (CH3CH2Pt). This is the first observation on the lifetime of surface ethyl species under reaction conditions. The rise of the final ethane product, monitored by the ν(CH) absorption at 2893 cm-1 was found to reach a maximum already in the first recorded time slice. This suggests that the observed CH3CH2Pt species is a surface-trapped form of the kinetically relevant, only weakly interacting C2H5 radical intermediate.