Control of the single atom/nanoparticle ratio in Pd/C catalysts to optimize the cooperative hydrogenation of alkenes

Abstract

We recently reported (R. Castro Contreras, B. Guicheret, B. F. Machado, C. Rivera-Cárcamo, M. A. Curiel Alvarez, B. Valdez Salas, M. Ruttert, T. Placke, A. Favre Réguillon, L. Vanoye, C. de Bellefon, R. Philippe and P. Serp, J. Catal., 2019, 372, 226–244) that a structure/activity correlation exists in Pd/C catalysts for myrcene hydrogenation, which integrates the Pd dispersion, and the surface concentration of oxygen groups and defects of the support. Here, through a combined experimental–theoretical study, we provide an explanation of the influence of these three structural characteristics of Pd/C catalysts for alkene hydrogenation. Highly dispersed Pd nanoparticles (Pd_NP) are necessary to activate dihydrogen. A high concentration of surface defects on the support is necessary to stabilize Pd single atoms (Pd_SA), which coexist with Pd_NP on Pd/C catalysts. A high concentration of oxygenated surface groups is also necessary on the support to allow hydrogen spillover. We demonstrate that such a combination allows cooperative catalysis to operate between Pd_NP and Pd_SA that involves the formation of Pd_SA–H species, which are much more active than Pd_NP–H for alkene hydrogenation but also isomerization. Importantly, we also report an efficient method to control the ratio between Pd_SA and Pd_NP in Pd/C catalysts of similar loadings and show that the control of this ratio allows the development of a new generation of stable and highly active catalysts integrating the ultra-rational use of precious metals in short supply. Indeed, for myrcene hydrogenation, activity variations of several orders of magnitude were measured as a function of the value of this ratio.