Catalytic properties of Al 13 TM 4 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- 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.