Selective semi-hydrogenation of acetylene: Atomistic scenario for reactions on the polar threefold surfaces of GaPd

The activity and selectivity of the polar threefold surfaces of the B20-type intermetallic compound GaPd for the semi-hydrogenation of acetylene to ethylene has been investigated using ab initio density functional theory (DFT). Because of the lack of inversion symmetry, the threefold (1 1 1) surfaces of the B20 structure have polar character, in both nonequivalent [I 1 1] and [(1) over bar (1) over bar (1) over bar] directions several surface terminations differing in structure and chemical composition are possible. The structural and energetic properties and the chemical reactivities of ten conceivable terminations have recently been investigated using DFf (Krajci and Hafner, 2013)-here detailed atomistic scenarios for the selective semi-hydrogenation of acetylene to ethylene on these surfaces have been developed. Threefold surfaces with only Ga-atoms in the outermost layer are energetically favorable at the Ga-rich end of the interval of the admissible chemical potentials, but they are catalytically inactive because molecular hydrogen cannot be dissociated and hence the hydrogenation of adsorbed hydrocarbon molecules is impossible. Highly corrugated surfaces with Pd atoms in the top atomic plane can hydrogenate adsorbed acetylene molecules easily. But simultaneously their high reactivity with respect to both molecular and atomic hydrogen complicates dissociation and diffusion processes. The selectivity of such surfaces is also insufficient as ethylene molecules adsorbed on-top of highly protruding Pd atoms can be easily attacked by co-adsorbed hydrogen atoms from the bottom. We have found that one of the possible threefold surface terminations with isolated Pd-3 triplets and Ga atoms in one-half of the hollows between the Pd3 triplets exhibits both superior catalytic activity and selectivity. However, this surface is energetically competitive only in a very narrow range of chemical potentials. An important factor for achieving a high selectivity is the transition from di-sigma bonded adsorption configuration for acetylene to a pi-bonded adsorption of ethylene. This transition has also been found in our earlier investigation of the (210) surface of GaPd as a catalyst for semi-hydrogenation (Kraci and Hafner, 2012). The large distances between the isolated active sites is also important to avoid undesired side-reactions such as oligomerization. Our results are well compatible with the available experimental observations. We have demonstrated that the observed activity and selectivity of GaPd catalyst originate form contributions from different surfaces terminations differing in activity and selectivity. An optimal selectivity of the catalyst can be achieved only if the formation of surfaces with low selectivity is avoided and terminations with enhanced selectivity are formed.