Distinct Kinetic Signatures of Photodesorption from Metal Nanoparticles

Visible photon fluxes can influence the rate and selectivity of heterogeneously catalyzed reactions on metal nanoparticle surfaces. Models describing the influence of photon fluxes have typically introduced photon flux dependent apparent thermal kinetic parameters (reaction orders, activation energies, binding energies, etc.). This has involved empirical fitting of reaction rate data, making mechanistic interpretations of the influence of photon fluxes on elementary step rates challenging, and is inconsistent with fundamental descriptions of photochemistry on metal surfaces developed from surface science studies. Using the CO adsorption-desorption quasi-equilibrium reaction on Pt/Al2O3 catalysts as a model “reaction” system, we measured steady-state adsorbed CO (CO*) coverages under isothermal and isobaric (1 mbar CO) conditions as a function temperature (473 – 573 K) and of 440 nm photon flux (0.1-5.2 x 103 #hv Pt site-1 s-1) using in-situ IR spectroscopy. Steady state CO* coverage on Pt was strongly photon flux dependent with increasing photon flux causing decreasing coverage, consistent with photons driving CO* desorption rates faster than thermal CO* desorption rates. However, photon flux dependent CO* coverages were essentially temperature independent, inconsistent with models that describe photon effects using perturbations to apparent thermal kinetic parameters. Instead, 120 steady state CO* coverages as a function of temperature and photon flux are quantitatively described by a kinetic model in which the overall desorption rate is a summation of independent thermal and photon-induced CO* desorption rates. Site-resolved analysis reveals distinct kinetic parameters for the photon driven desorption of CO* from well-coordinated, under-coordinated, and highly under-coordinated Pt sites, with temperature-dependent apparent quantum efficiencies (AQE) consistent with temperature dependence of vibrational quanta distribution of CO. The rigorous kinetic rate laws for independent photon and thermal driven pathways allows for predictive modeling of operating regimes where photon fluxes can drive enhance reaction rates. This work provides evidence that steady state continuous wave photon fluxes can drive reactions on metal surfaces out of thermal equilibrium, reconciles surface science observations of molecular photodesorption with applied catalysis, establishes a general kinetic framework for photon-driven processes on metals, and defines catalyst, reaction, and photon flux characteristic design principles for breaking Sabatier limitations.