(710e) Pt-Based Single Atom Alloys for Citral Hydrogenation

The selective hydrogenation of α,β-unsaturated aldehydes is used in the production of various fine chemicals. In this work, we investigate the selective hydrogenation of citral. This model reaction has been studied over a variety of different catalyst systems, however these catalysts typically contain a distribution of structures that make it challenging to understand the origin of selectivity trends. To elucidate active site structure-performance relationships, we have synthesized single atom alloy (SAA) catalysts.

SAA catalysts, comprised of one metal diluted in a host, are well-controlled materials which offer geometric and electronic modifications which may influence the catalytic performance. We synthesized SAA catalysts composed of Pt single atoms dispersed in Mn, Fe, Co, Ni, or Cu host nanoparticles. The series of metals spans a range of surface segregation energies, therefore, the Pt surface structure of these catalysts is expected to change across host metals. These catalysts were characterized using chemisorption, CO-FTIR, and STEM.

Citral hydrogenation was carried out over the host and SAA catalysts. The Ni/SiO₂ and NiPt/SiO₂ catalysts are the most active with rates of 10 and 29 mmol min^-1 g_metal^-1, respectively. The MnPt, FePt, CoPt and CuPt/SiO₂ catalysts have an order of magnitude less activity, between 0.98 and 1.33 mmol min^-1 g_metal^-1, which suggests that most of the catalytic activity is taking place on the dilute Pt sites which are similar across the catalysts. The Ni containing catalysts have a high selectivity to citronellol, while the Co and Cu containing catalysts are primarily selective to citronellal.

We observe rate enhancements and changes to the selectivity for citral hydrogenation for Pt containing SAA catalysts compared to monometallic host metal catalysts. These catalysts offer uniform active site structures which enable us to develop structure-performance relationships for citral hydrogenation and determine whether geometric or electronic modifications dominate the observed activity.