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

• Intermetallic Al13Fe4(0 1 0) is highly active for butadiene partial hydrogenation.
• With respect to Pd(1 0 0), a larger fraction of cis-2-butene is formed.
• The proposed reaction scheme involves adsorbed π-allyl and butyl intermediates.
• O-containing gas impurities chemisorb and form an AlOx layer causing slow deactivation.
• The surface can be fully regenerated through UHV annealing.

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(0 1 0) surface was investigated in the 0.2–2 kPa 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(1 0 0) 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.

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