Millisecond FT-IR Spectroscopy of Surface Intermediates of C₂H₄Hydrogenation over Pt/Al₂O₃Catalyst under Reaction Conditions

Abstract

Time-resolved rapid-scan FT-IR spectra of ethylene hydrogenation over alumina-supported Pt catalyst at 473 K were recorded under continuous H 2 /N 2 flow (1 atm) and pulsed release of C 2 H 4 (30 millisecond duration). Two surface species were observed, namely ethylidyne (CH 3 CPt 3 ) 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 C 2 H 4 + H 2 results with those of experiments using D 2 or C 2 D 4 suggests that the 1200 cm -1 species is a surface ethyl intermediate (CH 3 CH 2 Pt). 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 CH 3 CH 2 Pt species is a surface-trapped form of the kinetically relevant, only weakly interacting C 2 H 5 radical intermediate.

Direct Observation of Surface Ethyl to Ethane Interconversion upon C2H4 Hydrogenation over Pt/Al2O3 Catalyst by Time-Resolved FT-IR Spectroscopy

“…Figure 5 shows the decay kinetics of the transient 1200 cm -1 band assigned to surface ethyl species. 26 We find that the decay time of 122 ± 10 ms obtained from a single exponential fit (Figure 5b) In agreement with the known spectator role of ethylidyne, there is only a very small band observed at 1339 cm -1 immediately after the ethylene pulse (Figure 6a). The species grows in with a rise time of 150 ms, as shown in Figure 6b.…”

“…26 The most striking observations upon decrease of the reaction temperature are 23 We conclude that surface ethyl to ethane interconversion is the rate determining step of ethylene hydrogenation under reaction conditions. The fact that the The very large temperature dependence of the intensity ratio of the 2 surface species CH 3 CH 2 Pt and CH 3 CPt 3 is worth noting.…”

“…29 The ethylene gas phase bands are the strongest in the first time slice and then decrease, principally due to removal of the reactant from the infrared viewing zone by gas flow. 26 The kinetics of the ethylene decrease is displayed in Figure 2. The depletion observed at trace of Figure 3a) shows that the spectral subtraction reveals the ν 5 (CH) mode of ethane at 2954 cm -1 in addition to the 2893 cm -1 band already observed in the raw spectra.…”

“…In an attempt to obtain direct evidence for the kinetic relevancy of the ethyl intermediate 26 The rise of both intermediates was too fast to be resolved on the 100 ms time scale. As expected, the decay time of the CH 3 CH 2 Pt species (around the time limit of 100 ms) was faster than that of CH 3 CPt 3 (300 ms).…”

Dynamics of Propane in Silica Mesopores Formed upon Propylene Hydrogenation over Pt Nanoparticles by Time-Resolved FT-IR Spectroscopy

“…4,5 The decay of the C 2 H 5 Pt species was found to exhibit the same temporal behavior as the rise of ethane, demonstrating directly the kinetic significance of the intermediate. The…”