Site-selective Plasma-enhanced Desorption of CO from Pt Catalyst Particles

The prospect of driving heterogeneous chemistry using plasmas has great potential for the electrification of the chemical industry. These processes are not only compatible with intermittent, renewable energy sources, but they also enable reaction pathways that are otherwise inaccessible in thermally activated processes. While promising, the fundamental activation mechanisms at the plasma-catalyst interface remain poorly characterized and understood. Here we impinge a low-temperature nonthermal plasma onto alumina-supported Pt clusters with carbon monoxide (CO) chemisorbed onto the catalyst surface. We perform temperature-programmed desorption (TPD) measurements using IR diffuse reflectance and measure the effective binding energy (BE) between CO and Pt. TPD results suggest that even a low-power radio-frequency (RF) argon plasma reduces the effective BE significantly. Photochemical effects are negligible in this process because the photon flux from the plasma (~5×10^-5 W/cm²) is not sufficient to affect the kinetics of desorption. A significantly higher flux is needed to do so, as determined by laser irradiation measurements. Capacitive probe measurements indicate that reduction in BE scales linearly with the measured plasma density. We deconvolute the CO IR peak to resolve the contributions from binding to well-coordinated (WC) sites (terrace) and under-coordinated (UC) sites (step and edge), and find that plasma exposure has a stronger effect on the UC sites. Density functional theory calculations point to this being a result of both plasma-induced charging and electric fields at the Pt surface. This work highlights the uniqueness with which plasmas can tune surface chemistry.