Non-noble intermetallic compounds as selective butadiene-hydrogenation catalysts: Al13Fe4 vs Al13Co4
Résumé
Non-noble complex intermetallic compounds have shown promising properties as inexpensive catalyst alternatives to Pt-group metals for alkyne [1] and alkene [2-4] partial hydrogenation. In this work, the gas-phase hydrogenation of 1,3-butadiene over the Al13Fe4(010) and Al13Co4(100) surfaces was investigated in the 0.2-2 kPa range at 20-200 °C in a batch-type reactor coupled with an ultrahigh-vacuum setup allowing for Auger electron spectroscopy and low energy electron diffraction. The results were also compared with those obtained for Pd(100) in the same conditions.
Clean Al13Fe4(010) 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 [2,3]. 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. This result can be explained in terms of active site isolation and steric constraints upon π-allylic precursors to 2-butenes [3]. The sensitivity of the Al-Fe surface to oxygen-containing gas impurities, forming an aluminum oxide overlayer at high coverage, leads to gradual deactivation under reaction conditions, which is the main issue for the practical use of non-noble metal catalysts. However, the alloy surface can be fully regenerated through high-temperature annealing [3].
While both Al13Fe4 and Al13Co4 have been claimed to be efficient for partial acetylene hydrogenation [1], Al13Co4(100) appears more stable but much less active than Al13Fe4(010) for butadiene partial hydrogenation. Although the bulk structures of the two compounds are similar, their surface structures and compositions, as determined by a combination of surface-science experiments and ab initio calculations, substantially differ [5]. The influence of these structural differences will be discussed in the light of recent density-functional-theory calculations.
[1] M. Armbrüster et al., Nat. Mater. 11 (2012) 690
[2] L. Piccolo, Chem. Commun. 49 (2013) 9149
[3] L. Piccolo, L. Kibis, J. Catal. 332 (2015) 112
[4] L. Piccolo, L. Kibis, M.C. DeWeerd, E. Gaudry, J. Ledieu, V. Fournée, ChemCatChem 9 (2017) 2292
[5] E. Gaudry et al., Phys. Rev. B 94 (2016) 165406