Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study of ethyne hydrogenation on Pd/Al2O3

Preliminary characterisation studies are reported regarding a flow switching experiment combined with DRIFTS monitoring, which has been applied for the first time to the study of adsorption and hydrogenation of ethyne and ethene over: (1) a 0.02 wt.% Pd/αAl2O3 catalyst and (2) a 2 wt.% Pd/γAl2O3 catalyst (Johnson Matthey). The catalysts were pre-reduced under a flow of hydrogen, followed by flushing under helium at 120 °C or 200 °C. During adsorption of ethyne, a carbonaceous layer was formed on the catalyst in the early stages of the exposure of the catalyst to ethyne, but carbonaceous deposits were not observed during ethene adsorption under identical conditions of freshly reduced catalyst. From the infrared spectra, octane was tentatively identified as being present in the carbonaceous layer. Upon exposing the catalyst to five 5 cm3 ‘pulses’ of ethyne, sequential hydrogenation to ethene and ethane occurred during the first two pulses applied even in the absence of gas phase hydrogen, but decreased significantly in the three subsequent pulses. Possible sources of hydrogen for the reaction were retention by the catalyst during the pre-reduction or transfer from the carbonaceous layer. Upon heating the 2 wt.% catalyst to 200 °C in a flow of helium to remove residual hydrogen from the pre-reduction, then exposing it to ethyne, hydrogenation still occurred with an increased selectivity towards ethane, suggesting that the carbonaceous layer was the more likely source of hydrogen. Less ethane was formed over the 0.02 wt.% Pd/α-Al2O3 catalyst compared with the 2 wt.% Pd/γ-Al2O3, as the former catalyst is designed to prevent hydrogenation to the alkane through its low metal loading, optimal pore structure and Pd dispersion. The technique gives insights in to the reaction pathways and could potentially be used to derive kinetic and transport parameters for the reaction system.