Effect of Pt Particle Size and Electronic Properties on the Hydrogenation of Ethylene

Abstract

The hydrogenation of ethylene to ethane is a widely studied model reaction to investigate catalytic mechanisms. This reaction was historically regarded as structure-insensitive, until recent studies revealed that minor variations in the metal cluster size can affect both the geometric and electronic properties, which in turn affects the catalytic activity. Platinum (Pt) based catalysts on reducible oxides such as ceria (CeO2) have gained attention due to the support's ability to temper the electronic properties of the metal. Therefore, this study first investigates the variation in Pt nuclearity on reducible CeO2 and its effect on the electronic properties of the metal, as well as the catalytic activity for ethylene hydro- genation. Moreover, this study also investigates the catalytic activity for Pt particles of the same size with varying electronic properties, by varying the reducibility of CeO2, with the aim of decoupling the geometric and electronic effects. This work combines advanced char- acterization techniques such as in situ infrared spectroscopy, Raman spectroscopy, and in situ X-ray absorption spectroscopy with kinetic measurements to correlate metal nuclearity and electronic properties with catalytic activity. The results reveal a volcano-shaped curve between the catalytic activity and Pt particle size, with the peak catalyst achieving orders of magnitude greater in activity. With the use of kinetic order measurements, we propose that both competitive and noncompetitive Horiuti–Polanyi mechanisms contribute to ethylene hydrogenation. The increase in low-coordination sites with decreasing particle size likely promotes hydrogen binding and dissociation. Furthermore, by varying the electronic prop- erties on the same particle size, we find that lower electron density correlates with higher activity, a trend in activity similar to what was observed for the different Pt particle sizes. The lower electron density and more positively charged particles likely promote hydrogen adsorption without the overbinding of carbonaceous intermediates, that poison the catalyst surface. These results highlight the complex interplay between electronic and geometric properties and provide key insights on the role of particle size and electronic metal-support interactions, helping to fill the gaps in existing knowledge and contributing to the design of efficient hydrogenation catalysts.