Hidden reservoir dynamics in oxygen temperature-programmed desorption from rhodium: A two-layer kinetic Monte Carlo study

Abstract

Temperature-programmed desorption (TPD) spectra constrain coupled surface kinetics but do not necessarily identify a unique microscopic pathway. We develop a two-layer kinetic Monte Carlo model for oxygen desorption from rhodium, including reversible surface–subsurface exchange, subsurface diffusion, lateral interactions, and recombinative O2 desorption from adjacent surface atoms. The reference calculation produces a narrow maximum at 845 K, compared with approximately 820 K for the digitized high-exposure Rh(111) trace, with full widths at half maximum of approximately 34 and 29 K, respectively. The model underestimates the persistent experimental contribution above 1000 K. No temperature shifting or point-by-point fitting was applied. A tenfold reduction of the exchange-transmission coefficient leaves the 10 K-binned maximum at 845 K while reducing gross exchange from approximately 51 600 to 4570 events per trajectory. Thus, the peak requires sufficiently rapid reservoir communication but does not determine the exchange barrier or transmission coefficient separately. Controlled tests of gateway spacing, penetration criteria, surface mixing, lateral interactions, direct subsurface loss, and fcc/hcp site inequivalence delimit this conclusion. A direct contribution near 17% broadens and displaces the principal feature, whereas fcc/hcp inequivalence preserves the narrow peak and reservoir-mediated sequence. These results point to a kinetic bottleneck in access to irreversible O2 formation: reversible reservoir communication may be extensive, but a substantial bypass of surface recombination strongly degrades spectral focusing. The calculations establish the sufficiency and admissible range of an emergence-mediated mechanism without claiming an unique inversion of the experimental TPD spectrum.