Catalytic properties of Al₁₃TM₄complex intermetallics: influence of the transition metal and the surface orientation on butadiene hydrogenation

Abstract

Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al 13 Co 4 (100), m-Al 13 Fe 4 (010) and m-Al 13 Ru 4 (010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al 13 Co 4 (100) and o-Al 13 Co 4 (010). As a result, the following room-temperature activity order is determined: Al 13 Co 4 (100) < Al 13 Co 4 (010) < Al 13 Ru 4 (010) < Al 13 Fe 4 (010). Moreover, Al 13 Co 4 (010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al 13 Fe 4 (010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al 13 TM 4 surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al 13 TM 4 compounds.

Catalytic activation of a non-noble intermetallic surface through nanostructuration under hydrogenation conditions revealed by atomistic thermodynamics

“…The surface energy landscape is also found to be flatter, the range of butadiene adsorption energy differences being below 100 meV, using both the PBE and DFT-D3 functionals. The stronger adsorption of butadiene on Al 13 Fe 4 (010) than on Al 13 Co 4 (100), classically attributed to a decreased filling of the adsorbate-metal antibonding states, is indeed in agreement with Bader charge calculations (section S5) and the lower-energy d-band center in the former case: -1.41 eV for bulk Al 13 Fe 4 , -1.97 eV for bulk Al 13 Co 4 21 .…”

“…The stability phase diagrams of hydrogenated surfaces are drawn for three different pressures, simulating industrial catalysis (2 bar) 67 and UHV conditions (5 • 10 −10 mbar), as well as the reaction conditions of Ref. 21 (Fig. 2).…”

“…The most stable isolated adsorption structure is considered for butadiene (configurations are shown in the SI) and the gas phase contains H 2 and C 4 H x (x ∈ {6, 8}) in the ratio 10:1 as in Ref. 21 (Fig. 3).…”

“…Al 13 TM 4 (TM = Fe, Co) complex intermetallic compounds have recently shown unexpected performances in the partial hydrogenation of alkynes 2,19 and alkadienes 4,20,21 . The bulk and surface structures of these compounds, considered as low scale prototypes of Al-based decagonal quasicrystals, have been extensively studied since several years [22][23][24][25][26][27][28][29][30][31][32] .…”

“…Not only the surface atomic structure but also electronic effects associated with the nature of the transition metal are expected to influence the catalytic performances. A recent experimental investigation of the catalytic performances of quasi isostructural Al 13 Co 4 (100) and Al 13 Fe 4 (010) surfaces for butadiene partial hydrogenation has revealed that the initial butene-formation activity of Al 13 Fe 4 is 5-10 times higher than that of Al 13 Co 4 , while the selectivity to butene is maximum on the latter 21 . In the second step of this work, we rationalize the differences between these two systems, by determining their respective hydrogen adsorption and hydrogen-hydrocarbon co-adsorption properties under reaction conditions.…”