Unravelling synergistic effects in bi-metallic catalysts: deceleration of palladium–gold nanoparticle coarsening in the hydrogenation of cinnamaldehyde

Abstract

In this work, we demonstrate that the synergistic effect of PdAu nanoparticles (NPs) in hydrogenation reactions is not only related to high activity but also to their stability when compared to Pd mono-metallic NPs. To demonstrate this, a series of mono- and bi-metallic NPs: Pd, Pd_0.75Au_0.25, Pd_0.5Au_0.5, Pd_0.25Au_0.75 and Au in ionic liquid [C₄C₁Im][NTf₂] have been fabricated via a magnetron sputtering process. Bi-metallic NPs possess external shells enriched with Pd atoms that interact with [NTf₂]⁻ of the ionic liquid resulting in enhanced catalytic performance in hydrogenation of cinnamaldehyde compared to their mono-metallic counterparts. This is ascribed to their higher stability over 24 h of reaction, whilst the catalytic activity and selectivity are comparable for both catalysts. Using a bespoke kinetic model for in situ catalyst deactivation investigations and electron microscopy imaging at the nanoscale, we have shown that PdAu has a deactivation rate constant of 0.13 h⁻¹, compared to 0.33 h⁻¹ for Pd NPs, leaving 60% and 40% of available sites after the reaction, respectively. Beyond that, the kinetic model demonstrates that the reaction product has a strong stabilizing factor for bimetallic NPs against coarsening and deactivation, which is not the case for Pd NPs. In summary, our kinetic model enables the evaluation of the catalyst performance over the entire chemical reaction space, probing the contribution of each individual component of the reaction mixture and allowing the design of high-performance catalysts.