Remarkable NanoConfinement Effects on Equilibrated Reactions: Statistical-Mechanics Modeling Focused on Ir Dimerization Beneath Surface Sites in Pd–Ir Nanoparticles

Chemical equilibrium involving a small number of molecules inside a confined nanospace can exhibit considerable deviations from the macroscopic thermodynamic limit due to reduced mixing entropy, as was predicted in several of our works using statistical-mechanics partition-functions and the lattice-gas model (LGM). In particular, significant enhancements of the equilibrium extent and constant are generally anticipated in the case of exothermic reactions. The present work is a substantial extension of this exploration of the so-called “nanoconfinement entropic effect on chemical equilibrium” (NCECE), focusing now on several new issues: (i) general derivation and computations for addition reactions in the non-lattice model (NLM), including endergonic reactions exhibiting significantly weakened NCECE, (ii) comparison with effects predicted for dimerization reactions, for which a novel “inverse NCECE” is obtained for the endergonic range, (iii) a concrete system modeling of Ir dimerization in the core of Pd–Ir cuboctahedral nanoparticles using uniform bond energetics in the LGM versus the NLM. The latter reproduces quite accurately the NCECE effects obtained by the LGM, thus avoiding tedious combinatorial computations, and (iv) Ir dimerization at subsurface sites of the Pd nanoparticles in the framework of the LGM with a more elaborate coordination-dependent bond energetics. It should be noted that the latter subsurface compositional variations can affect catalytic properties of Pd–Ir nanoparticles such as those operating in several applications.