Platinum-Copper and Palladium-Gold Single Atom Alloys for Selective Hydrogenation Reactions

Selective hydrogenation of alkynes and dienes to alkenes is important for producing polymer-grade alkenes [1]. As an attractive alternative to conventional bimetallic catalysts, single atoms alloys (SAAs) can act as bifunctional surfaces with individual, isolated, highly active group VIII metal atoms dispersed in inert nanoparticles [2]. The SAAs are capable of dissociating and spilling over the hydrogen to the group IB noble metal areas that weakly adsorb hydrogen atoms and are active for selective hydrogenation reactions [2]. It was recently demonstrated that alloying of a trace amount of palladium in copper increases the reactivity markedly, while maintaining the high selectivity of copper for hydrogenation reactions [3].

Here we have investigated the selective hydrogenation performance of Pt-Cu and Pd-Au SAAs. Pt-Cu and Pd-Au nanoparticles enhance both the activity and selectivity for industrially important hydrogenation reactions for 1, 3 – butadiene and phenylacetylene when compared to their monometallic catalysts. We show both an increase in activity compared to monometallic Cu or Au and an increase in selectivity compared to monometallic Pt or Pd. The Pt atoms were loaded on pre-formed Cu nanoparticles with galvanic replacement reaction, while the Pd-Au bimetallic nanoparticles were formed by sequential deposition of palladium onto the gold surfaces. The preparation, reactivity, and characterization of the alloy nanoparticles with X-ray photoelectron spectroscopy, electron microscopy and X-ray absorption techniques will be presented. Using scanning tunneling microscopy (STM) and temperature programmed desorption/reaction (TPD/R), we show that Pt-Cu(111) SAAs catalyze hydrogenation of butadiene to butene with 100% selectivity. Pt atoms readily alloy into Cu(111) at low coverages and moderate temperatures where the Pt atoms exist as isolated atoms that are well separated from one another. The addition of small amounts of Pt (~1%) into Cu reduces the barrier for H₂ dissociation, allowing for the low temperature dissociation of H₂. Hydrogen atoms spill-over onto the Cu sites increasing the concentration of weakly bound H atoms that readily hydrogenate butadiene to butene. TPR of co-adsorbed H and butadiene shows the exclusive desorption of reactively formed butene, where the reaction extent is limited by the availability of H on the surface.

References

[1] Segura, Y., López, N. & Pérez-Ramírez, J. Origin of the superior hydrogenation selectivity of gold nanoparticles in alkyne- alkene mixtures: Triple-versus double-bond activation. Journal of Catalysis 247, 383-386 (2007).

[2] Kyriakou, G. et al. Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 335, 1209-1212 (2012).

[3] Boucher, M. B. et al. Single Atom Alloy Surface Analogs in Pd_0.18Cu₁₅ Nanoparticles for Selective Hydrogenation Reactions. Phys.Chem.Chem.Phys. 15, 12187-96 (2013).