Formation and stability of an active PdZn nanoparticle catalyst on a hydrotalcite-based support for ethanol dehydrogenation

Abstract

A hydrotalcite-based PdZn nanoparticle catalyst, PdZn/Mg(Al)(Pd)(Zn)O_x has been synthesized via a one-pot procedure. The activation comprising H₂ and air treatment(s) allows tuning the nanoparticle formation and, hence, the catalyst performance. Based on an elaborate set of characterization data from EXAFS, in situ XRD, STEM and CO chemisorption, it is concluded that single reduction leads to the formation of Pd-rich alloy nanoparticles, i.e., a PdZn shell with a Pd core. Cycled reduction, i.e., 3 subsequent hydrogen and air treatments, ensures the formation of more homogeneously mixed PdZn nanoparticles. Compared with a PdZn/ZnO reference catalyst, the nanoparticles obtained after cycled reduction exhibit a higher initial average turnover frequency in ethanol dehydrogenation, i.e., 7.0 molEtOH (mol_Pd s)⁻¹ rather than 3.2 molEtOH (mol_Pd s)⁻¹. An activity loss is observed during the first hours on stream. It is attributed to coking of the Pd sites which are also deemed responsible for acetaldehyde decomposition. Hence, the acetaldehyde selectivity steadily increases during the first hours on stream. Subsequently, the acetaldehyde space time yield and selectivity stabilize at 0.7 × 10⁻⁴ mol s⁻¹ kg⁻¹_Pd and 98%, respectively.