Catalytic properties of Al13TM4 complex intermetallics: influence of the transition metal and the surface orientation on butadiene hydrogenation.

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-Al₁₃Co₄(100), m-Al₁₃Fe₄(010) and m-Al₁₃Ru₄(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-Al₁₃Co₄(100) and o-Al₁₃Co₄(010). As a result, the following room-temperature activity order is determined: Al₁₃Co₄(100) < Al₁₃Co₄(010) < Al₁₃Ru₄(010) < Al₁₃Fe₄(010). Moreover, Al₁₃Co₄(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al₁₃Fe₄(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 Al₁₃TM₄ 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 Al₁₃TM₄ compounds. [ABSTRACT FROM AUTHOR]