Catalytic properties of AlTM 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 AlCo(100), -AlFe(010) and -AlRu(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 AlCo(100) and AlCo(010). As a result, the following room-temperature activity order is determined: AlCo(100) < AlCo(010) < AlRu(010) < AlFe(010). Moreover, AlCo(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated AlFe(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 AlTM 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 AlTM compounds.