The partial hydrogenation of butadiene over Al13Fe4: A surface-science study of reaction and deactivation mechanisms

Abstract

Non-noble intermetallic compounds have shown promising properties as inexpensive catalyst alternatives to Pt-group metals for alkyne and alkene hydrogenation. In this work, the gas-phase hydrogenation of 1,3-butadiene over the Al13Fe4(010) surface was investigated in the 0.2–2kPa total pressure range at 20–200°C in a batch-type reactor coupled with an ultrahigh-vacuum setup allowing for Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The results were compared with those previously obtained on Pd(100) in the same conditions. It is confirmed that Al–Fe is initially as active as Pd and 100% selective to butenes, including at room temperature (RT), with sequential conversions of butadiene to butenes, and butenes to butane. The main difference with Pd comes from the butenes distribution, with a cis/trans 2-butene ratio larger than unity for Al–Fe while it is near zero for Pd. The results are discussed in terms of (i) steric constraints upon π-allylic precursors to 2-butenes and (ii) involvement of adsorbed butyl intermediates allowing for hydro-isomerization of butenes competing with their hydrogenation to butane. A mechanistic reaction scheme is proposed accordingly. The sensitivity of the Al–Fe surface to oxygen-containing impurities leads to gradual deactivation under reaction conditions, which is the main issue for practical use of non-noble metal catalysts. The deactivation and oxidation processes were investigated by combining post-reaction AES measurements with several thermal/chemical treatments. Depending on the pressure conditions, the Al13Fe4 surface chemisorbs oxygen-containing species or forms an Al oxide layer. The RT activity of the surface decreases as the oxygen-containing phase coverage increases. However, this phase can be removed through high-temperature annealing.